feat: send pre-message on messages with large attachments (#7410)

part of #7367

progress / what's to do:
- [x] send pre-message
- [x] The message's state must be set to MessageState::OutDelivered only
after both messages are sent. If a read receipt is received, the message
can be OutMdnRcvd or OutPending; let's just do whatever is easiest for
now. Take care not to revert from OutMdnReceived to OutDelivered if we
first receive a read receipt and then deliver the full message.
   - this is already the case:
- `OutDelivered` is set when a message is sent out and has no remaining
send jobs in the smtp table for this message id - so already works since
full message and pre message have same msgId in that table.
- `OutMdnRcvd` is a "virtual" state
(https://github.com/chatmail/core/issues/7367#issuecomment-3500891040),
so going back to `OutDelivered` can't happen anymore
- [x] delimit `ChatFullMessageId` with `<` and `>` like the other
message ids
- [x] add tests
	- [x] test that pre message is sent for attachment larger than X
		- test that correct headers are present on both messages
- also test that Autocrypt-gossip and selfavatar should never go into
full-messages
	- [x] test that no pre message is sent for attachment smaller than X
		- no "is full message" header should be present
- [x] test that pre message is not send for large webxdc update or large
text
- [x] fix test
`receive_imf::receive_imf_tests::test_dont_reverify_by_self_on_outgoing_msg`

---------

Co-authored-by: iequidoo <117991069+iequidoo@users.noreply.github.com>
Co-authored-by: Hocuri <hocuri@gmx.de>

src/chat.rs

@@ -27,7 +27,9 @@ use crate::constants::{
 use crate::contact::{self, Contact, ContactId, Origin};
 use crate::context::Context;
 use crate::debug_logging::maybe_set_logging_xdc;
-use crate::download::DownloadState;
+use crate::download::{
+    DownloadState, PRE_MSG_ATTACHMENT_SIZE_THRESHOLD, PRE_MSG_SIZE_WARNING_THRESHOLD,
+};
 use crate::ephemeral::{Timer as EphemeralTimer, start_chat_ephemeral_timers};
 use crate::events::EventType;
 use crate::key::self_fingerprint;
@@ -35,7 +37,7 @@ use crate::location;
 use crate::log::{LogExt, warn};
 use crate::logged_debug_assert;
 use crate::message::{self, Message, MessageState, MsgId, Viewtype};
-use crate::mimefactory::MimeFactory;
+use crate::mimefactory::{MimeFactory, RenderedEmail};
 use crate::mimeparser::SystemMessage;
 use crate::param::{Param, Params};
 use crate::receive_imf::ReceivedMsg;
@@ -2728,6 +2730,52 @@ async fn prepare_send_msg(
     Ok(row_ids)
 }
 
+/// Renders the message or Full-Message and Pre-Message.
+///
+/// Pre-Message is a small message with metadata which announces a larger Full-Message.
+/// Full messages are not downloaded in the background.
+///
+/// If pre-message is not nessesary this returns a normal message instead.
+async fn render_mime_message_and_pre_message(
+    context: &Context,
+    msg: &mut Message,
+    mimefactory: MimeFactory,
+) -> Result<(RenderedEmail, Option<RenderedEmail>)> {
+    let needs_pre_message = msg.viewtype.has_file()
+        && mimefactory.will_be_encrypted() // unencrypted is likely email, we don't want to spam by sending multiple messages
+        && msg
+            .get_filebytes(context)
+            .await?
+            .context("filebytes not available, even though message has attachment")?
+            > PRE_MSG_ATTACHMENT_SIZE_THRESHOLD;
+
+    if needs_pre_message {
+        let mut mimefactory_full_msg = mimefactory.clone();
+        mimefactory_full_msg.set_as_full_message();
+        let rendered_msg = mimefactory_full_msg.render(context).await?;
+
+        let mut mimefactory_pre_msg = mimefactory;
+        mimefactory_pre_msg.set_as_pre_message_for(&rendered_msg);
+        let rendered_pre_msg = mimefactory_pre_msg
+            .render(context)
+            .await
+            .context("pre-message failed to render")?;
+
+        if rendered_pre_msg.message.len() > PRE_MSG_SIZE_WARNING_THRESHOLD {
+            warn!(
+                context,
+                "Pre-message for message (MsgId={}) is larger than expected: {}.",
+                msg.id,
+                rendered_pre_msg.message.len()
+            );
+        }
+
+        Ok((rendered_msg, Some(rendered_pre_msg)))
+    } else {
+        Ok((mimefactory.render(context).await?, None))
+    }
+}
+
 /// Constructs jobs for sending a message and inserts them into the appropriate table.
 ///
 /// Updates the message `GuaranteeE2ee` parameter and persists it
@@ -2799,7 +2847,8 @@ pub(crate) async fn create_send_msg_jobs(context: &Context, msg: &mut Message) -
         return Ok(Vec::new());
     }
 
-    let rendered_msg = match mimefactory.render(context).await {
+    let (rendered_msg, rendered_pre_msg) =
+        match render_mime_message_and_pre_message(context, msg, mimefactory).await {
             Ok(res) => Ok(res),
             Err(err) => {
                 message::set_msg_failed(context, msg, &err.to_string()).await?;
@@ -2864,12 +2913,26 @@ pub(crate) async fn create_send_msg_jobs(context: &Context, msg: &mut Message) -
         } else {
             for recipients_chunk in recipients.chunks(chunk_size) {
                 let recipients_chunk = recipients_chunk.join(" ");
+                // send pre-message before actual message
+                if let Some(pre_msg) = &rendered_pre_msg {
                     let row_id = t.execute(
-                    "INSERT INTO smtp (rfc724_mid, recipients, mime, msg_id) \
+                        "INSERT INTO smtp (rfc724_mid, recipients, mime, msg_id)
+                        VALUES            (?1,         ?2,         ?3,   ?4)",
+                        (
+                            &pre_msg.rfc724_mid,
+                            &recipients_chunk,
+                            &pre_msg.message,
+                            msg.id,
+                        ),
+                    )?;
+                    row_ids.push(row_id.try_into()?);
+                }
+                let row_id = t.execute(
+                    "INSERT INTO smtp (rfc724_mid, recipients, mime, msg_id)
                     VALUES            (?1,         ?2,         ?3,   ?4)",
                     (
                         &rendered_msg.rfc724_mid,
-                        recipients_chunk,
+                        &recipients_chunk,
                         &rendered_msg.message,
                         msg.id,
                     ),

src/download.rs

@@ -17,6 +17,22 @@ use crate::{EventType, chatlist_events};
 /// `MIN_DELETE_SERVER_AFTER` increases the timeout in this case.
 pub(crate) const MIN_DELETE_SERVER_AFTER: i64 = 48 * 60 * 60;
 
+/// From this point onward outgoing messages are considered large
+/// and get a pre-message, which announces the full message.
+// this is only about sending so we can modify it any time.
+// current value is a bit less than the minimum auto download setting from the UIs (which is 160 KiB)
+pub(crate) const PRE_MSG_ATTACHMENT_SIZE_THRESHOLD: u64 = 140_000;
+
+/// Max message size to be fetched in the background.
+/// This limit defines what messages are fully fetched in the background.
+/// This is for all messages that don't have the Chat-Is-Full-Message header.
+#[allow(unused)]
+pub(crate) const MAX_FETCH_MSG_SIZE: usize = 1_000_000;
+
+/// Max size for pre messages. A warning is emitted when this is exceeded.
+/// Should be well below `MAX_FETCH_MSG_SIZE`
+pub(crate) const PRE_MSG_SIZE_WARNING_THRESHOLD: usize = 150_000;
+
 /// Download state of the message.
 #[derive(
     Debug,
@@ -192,12 +208,17 @@ impl Session {
 
 #[cfg(test)]
 mod tests {
+    use mailparse::MailHeaderMap;
     use num_traits::FromPrimitive;
+    use tokio::fs;
 
     use super::*;
-    use crate::chat::send_msg;
+    use crate::chat::{self, create_group, send_msg};
+    use crate::config::Config;
+    use crate::headerdef::{HeaderDef, HeaderDefMap};
+    use crate::message::Viewtype;
     use crate::receive_imf::receive_imf_from_inbox;
-    use crate::test_utils::TestContext;
+    use crate::test_utils::{self, TestContext, TestContextManager};
 
     #[test]
     fn test_downloadstate_values() {
@@ -294,4 +315,327 @@ mod tests {
 
         Ok(())
     }
+    /// Tests that pre message is sent for attachment larger than `PRE_MSG_ATTACHMENT_SIZE_THRESHOLD`
+    /// Also test that pre message is sent first, before the full message
+    /// And that Autocrypt-gossip and selfavatar never go into full-messages
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_sending_pre_message() -> Result<()> {
+        let mut tcm = TestContextManager::new();
+        let alice = &tcm.alice().await;
+        let bob = &tcm.bob().await;
+        let fiona = &tcm.fiona().await;
+        let group_id = alice
+            .create_group_with_members("test group", &[bob, fiona])
+            .await;
+
+        let mut msg = Message::new(Viewtype::File);
+        msg.set_file_from_bytes(alice, "test.bin", &[0u8; 300_000], None)?;
+        msg.set_text("test".to_owned());
+
+        // assert that test attachment is bigger than limit
+        assert!(msg.get_filebytes(alice).await?.unwrap() > PRE_MSG_ATTACHMENT_SIZE_THRESHOLD);
+
+        let msg_id = chat::send_msg(alice, group_id, &mut msg).await?;
+        let smtp_rows = alice.get_smtp_rows_for_msg(msg_id).await;
+
+        //   pre-message and full message should be present
+        //   and test that correct headers are present on both messages
+        assert_eq!(smtp_rows.len(), 2);
+        let pre_message = smtp_rows.first().expect("first element exists");
+        let pre_message_parsed = mailparse::parse_mail(pre_message.payload.as_bytes())?;
+        let full_message = smtp_rows.get(1).expect("second element exists");
+        let full_message_parsed = mailparse::parse_mail(full_message.payload.as_bytes())?;
+
+        assert!(
+            pre_message_parsed
+                .headers
+                .get_first_header(HeaderDef::ChatIsFullMessage.get_headername())
+                .is_none()
+        );
+        assert!(
+            full_message_parsed
+                .headers
+                .get_first_header(HeaderDef::ChatIsFullMessage.get_headername())
+                .is_some()
+        );
+
+        assert_eq!(
+            full_message_parsed
+                .headers
+                .get_header_value(HeaderDef::MessageId),
+            Some(format!("<{}>", msg.rfc724_mid)),
+            "full message should have the rfc message id of the database message"
+        );
+
+        assert_ne!(
+            pre_message_parsed
+                .headers
+                .get_header_value(HeaderDef::MessageId),
+            full_message_parsed
+                .headers
+                .get_header_value(HeaderDef::MessageId),
+            "message ids of pre message and full message should be different"
+        );
+
+        let decrypted_full_message = bob.parse_msg(full_message).await;
+        assert_eq!(decrypted_full_message.decrypting_failed, false);
+        assert_eq!(
+            decrypted_full_message.header_exists(HeaderDef::ChatFullMessageId),
+            false
+        );
+
+        let decrypted_pre_message = bob.parse_msg(pre_message).await;
+        assert_eq!(
+            decrypted_pre_message
+                .get_header(HeaderDef::ChatFullMessageId)
+                .map(String::from),
+            full_message_parsed
+                .headers
+                .get_header_value(HeaderDef::MessageId)
+        );
+        assert!(
+            pre_message_parsed
+                .headers
+                .get_header_value(HeaderDef::ChatFullMessageId)
+                .is_none(),
+            "no Chat-Full-Message-ID header in unprotected headers of Pre-Message"
+        );
+
+        Ok(())
+    }
+
+    /// Tests that pre message has autocrypt gossip headers and self avatar
+    /// and full message doesn't have these headers
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_selfavatar_and_autocrypt_gossip_goto_pre_message() -> Result<()> {
+        let mut tcm = TestContextManager::new();
+        let alice = &tcm.alice().await;
+        let bob = &tcm.bob().await;
+        let fiona = &tcm.fiona().await;
+        let group_id = alice
+            .create_group_with_members("test group", &[bob, fiona])
+            .await;
+
+        let mut msg = Message::new(Viewtype::File);
+        msg.set_file_from_bytes(alice, "test.bin", &[0u8; 300_000], None)?;
+        msg.set_text("test".to_owned());
+
+        // assert that test attachment is bigger than limit
+        assert!(msg.get_filebytes(alice).await?.unwrap() > PRE_MSG_ATTACHMENT_SIZE_THRESHOLD);
+
+        // simulate conditions for sending self avatar
+        let avatar_src = alice.get_blobdir().join("avatar.png");
+        fs::write(&avatar_src, test_utils::AVATAR_900x900_BYTES).await?;
+        alice
+            .set_config(Config::Selfavatar, Some(avatar_src.to_str().unwrap()))
+            .await?;
+
+        let msg_id = chat::send_msg(alice, group_id, &mut msg).await?;
+        let smtp_rows = alice.get_smtp_rows_for_msg(msg_id).await;
+
+        assert_eq!(smtp_rows.len(), 2);
+        let pre_message = smtp_rows.first().expect("first element exists");
+        let full_message = smtp_rows.get(1).expect("second element exists");
+        let full_message_parsed = mailparse::parse_mail(full_message.payload.as_bytes())?;
+
+        let decrypted_pre_message = bob.parse_msg(pre_message).await;
+        assert!(
+            decrypted_pre_message
+                .get_header(HeaderDef::ChatFullMessageId)
+                .is_some(),
+            "tested message is not a pre-message, sending order may be broken"
+        );
+        assert_ne!(decrypted_pre_message.gossiped_keys.len(), 0);
+        assert_ne!(decrypted_pre_message.user_avatar, None);
+
+        let decrypted_full_message = bob.parse_msg(full_message).await;
+        assert!(
+            full_message_parsed
+                .headers
+                .get_first_header(HeaderDef::ChatIsFullMessage.get_headername())
+                .is_some(),
+            "tested message is not a full-message, sending order may be broken"
+        );
+        assert_eq!(decrypted_full_message.gossiped_keys.len(), 0);
+        assert_eq!(decrypted_full_message.user_avatar, None);
+        Ok(())
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_unecrypted_gets_no_pre_message() -> Result<()> {
+        let mut tcm = TestContextManager::new();
+        let alice = &tcm.alice().await;
+
+        let chat = alice
+            .create_chat_with_contact("example", "email@example.org")
+            .await;
+
+        let mut msg = Message::new(Viewtype::File);
+        msg.set_file_from_bytes(alice, "test.bin", &[0u8; 300_000], None)?;
+        msg.set_text("test".to_owned());
+
+        let msg_id = chat::send_msg(alice, chat.id, &mut msg).await?;
+        let smtp_rows = alice.get_smtp_rows_for_msg(msg_id).await;
+
+        assert_eq!(smtp_rows.len(), 1);
+        let message_bytes = smtp_rows
+            .first()
+            .expect("first element exists")
+            .payload
+            .as_bytes();
+        let message = mailparse::parse_mail(message_bytes)?;
+        assert!(
+            message
+                .headers
+                .get_first_header(HeaderDef::ChatIsFullMessage.get_headername())
+                .is_none(),
+        );
+        Ok(())
+    }
+
+    /// Tests that no pre message is sent for normal message
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_not_sending_pre_message_no_attachment() -> Result<()> {
+        let mut tcm = TestContextManager::new();
+        let alice = &tcm.alice().await;
+        let bob = &tcm.bob().await;
+        let chat = alice.create_chat(bob).await;
+
+        // send normal text message
+        let mut msg = Message::new(Viewtype::Text);
+        msg.set_text("test".to_owned());
+        let msg_id = chat::send_msg(alice, chat.id, &mut msg).await.unwrap();
+        let smtp_rows = alice.get_smtp_rows_for_msg(msg_id).await;
+
+        assert_eq!(smtp_rows.len(), 1, "only one message should be sent");
+
+        let msg = smtp_rows.first().expect("first element exists");
+        let mail = mailparse::parse_mail(msg.payload.as_bytes())?;
+
+        assert!(
+            mail.headers
+                .get_first_header(HeaderDef::ChatIsFullMessage.get_headername())
+                .is_none(),
+            "no 'Chat-Is-Full-Message'-header should be present"
+        );
+        assert!(
+            mail.headers
+                .get_first_header(HeaderDef::ChatFullMessageId.get_headername())
+                .is_none(),
+            "no 'Chat-Full-Message-ID'-header should be present in clear text headers"
+        );
+        let decrypted_message = bob.parse_msg(msg).await;
+        assert!(
+            !decrypted_message.header_exists(HeaderDef::ChatFullMessageId),
+            "no 'Chat-Full-Message-ID'-header should be present"
+        );
+
+        // test that pre message is not send for large large text
+        let mut msg = Message::new(Viewtype::Text);
+        let long_text = String::from_utf8(vec![b'a'; 300_000])?;
+        assert!(long_text.len() > PRE_MSG_ATTACHMENT_SIZE_THRESHOLD.try_into().unwrap());
+        msg.set_text(long_text);
+        let msg_id = chat::send_msg(alice, chat.id, &mut msg).await.unwrap();
+        let smtp_rows = alice.get_smtp_rows_for_msg(msg_id).await;
+
+        assert_eq!(smtp_rows.len(), 1, "only one message should be sent");
+
+        let msg = smtp_rows.first().expect("first element exists");
+        let mail = mailparse::parse_mail(msg.payload.as_bytes())?;
+
+        assert!(
+            mail.headers
+                .get_first_header(HeaderDef::ChatIsFullMessage.get_headername())
+                .is_none()
+        );
+        assert!(
+            mail.headers
+                .get_first_header(HeaderDef::ChatFullMessageId.get_headername())
+                .is_none(),
+            "no 'Chat-Full-Message-ID'-header should be present in clear text headers"
+        );
+        let decrypted_message = bob.parse_msg(msg).await;
+        assert!(
+            !decrypted_message.header_exists(HeaderDef::ChatFullMessageId),
+            "no 'Chat-Full-Message-ID'-header should be present"
+        );
+        Ok(())
+    }
+
+    /// Tests that no pre message is sent for attachment smaller than `PRE_MSG_ATTACHMENT_SIZE_THRESHOLD`
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_not_sending_pre_message_for_small_attachment() -> Result<()> {
+        let mut tcm = TestContextManager::new();
+        let alice = &tcm.alice().await;
+        let bob = &tcm.bob().await;
+        let chat = alice.create_chat(bob).await;
+
+        let mut msg = Message::new(Viewtype::File);
+        msg.set_file_from_bytes(alice, "test.bin", &[0u8; 100_000], None)?;
+        msg.set_text("test".to_owned());
+
+        // assert that test attachment is smaller than limit
+        assert!(msg.get_filebytes(alice).await?.unwrap() < PRE_MSG_ATTACHMENT_SIZE_THRESHOLD);
+
+        let msg_id = chat::send_msg(alice, chat.id, &mut msg).await.unwrap();
+        let smtp_rows = alice.get_smtp_rows_for_msg(msg_id).await;
+
+        //   only one message and no "is full message" header should be present
+        assert_eq!(smtp_rows.len(), 1);
+
+        let msg = smtp_rows.first().expect("first element exists");
+        let mail = mailparse::parse_mail(msg.payload.as_bytes())?;
+
+        assert!(
+            mail.headers
+                .get_first_header(HeaderDef::ChatIsFullMessage.get_headername())
+                .is_none()
+        );
+        assert!(
+            mail.headers
+                .get_first_header(HeaderDef::ChatFullMessageId.get_headername())
+                .is_none(),
+            "no 'Chat-Full-Message-ID'-header should be present in clear text headers"
+        );
+        let decrypted_message = bob.parse_msg(msg).await;
+        assert!(
+            !decrypted_message.header_exists(HeaderDef::ChatFullMessageId),
+            "no 'Chat-Full-Message-ID'-header should be present"
+        );
+
+        Ok(())
+    }
+
+    /// Tests that pre message is not send for large webxdc updates
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_render_webxdc_status_update_object_range() -> Result<()> {
+        let t = TestContext::new_alice().await;
+        let chat_id = create_group(&t, "a chat").await?;
+
+        let instance = {
+            let mut instance = Message::new(Viewtype::File);
+            instance.set_file_from_bytes(
+                &t,
+                "minimal.xdc",
+                include_bytes!("../test-data/webxdc/minimal.xdc"),
+                None,
+            )?;
+            let instance_msg_id = send_msg(&t, chat_id, &mut instance).await?;
+            assert_eq!(instance.viewtype, Viewtype::Webxdc);
+            Message::load_from_db(&t, instance_msg_id).await
+        }
+        .unwrap();
+
+        t.pop_sent_msg().await;
+        assert_eq!(t.sql.count("SELECT COUNT(*) FROM smtp", ()).await?, 0);
+
+        let long_text = String::from_utf8(vec![b'a'; 300_000])?;
+        assert!(long_text.len() > PRE_MSG_ATTACHMENT_SIZE_THRESHOLD.try_into().unwrap());
+        t.send_webxdc_status_update(instance.id, &format!("{{\"payload\": \"{long_text}\"}}"))
+            .await?;
+        t.flush_status_updates().await?;
+
+        assert_eq!(t.sql.count("SELECT COUNT(*) FROM smtp", ()).await?, 1);
+        Ok(())
+    }
 }

src/headerdef.rs

@@ -102,6 +102,17 @@ pub enum HeaderDef {
     /// used to encrypt and decrypt messages.
     /// This secret is sent to a new member in the member-addition message.
     ChatBroadcastSecret,
+    /// A message with a large attachment is split into two MIME messages:
+    /// A pre-message, which contains everything but the attachment,
+    /// and a full-message.
+    /// The pre-message gets a `Chat-Full-Message-Id` header
+    /// referencing the full-message's rfc724_mid.
+    ChatFullMessageId,
+
+    /// This message is preceded by a pre-message
+    /// and thus this message can be skipped while fetching messages.
+    /// This is a cleartext / unproteced header.
+    ChatIsFullMessage,
 
     /// [Autocrypt](https://autocrypt.org/) header.
     Autocrypt,
@@ -147,6 +158,9 @@ pub enum HeaderDef {
 
 impl HeaderDef {
     /// Returns the corresponding header string.
+    ///
+    /// Format is lower-kebab-case for easy comparisons.
+    /// This method is used in message receiving and testing.
     pub fn get_headername(&self) -> &'static str {
         self.into()
     }

src/mimefactory.rs

@@ -59,6 +59,15 @@ pub enum Loaded {
     },
 }
 
+#[derive(Debug, Clone, PartialEq)]
+pub enum PreMessageMode {
+    /// adds the Chat-Is-Full-Message header in unprotected part
+    FullMessage,
+    /// adds the Chat-Full-Message-ID header to protected part
+    /// also adds metadata and explicitly excludes attachment
+    PreMessage { full_msg_rfc724_mid: String },
+}
+
 /// Helper to construct mime messages.
 #[derive(Debug, Clone)]
 pub struct MimeFactory {
@@ -146,6 +155,9 @@ pub struct MimeFactory {
 
     /// This field is used to sustain the topic id of webxdcs needed for peer channels.
     webxdc_topic: Option<TopicId>,
+
+    /// This field is used when this is either a pre-message or a full-message.
+    pre_message_mode: Option<PreMessageMode>,
 }
 
 /// Result of rendering a message, ready to be submitted to a send job.
@@ -500,6 +512,7 @@ impl MimeFactory {
             sync_ids_to_delete: None,
             attach_selfavatar,
             webxdc_topic,
+            pre_message_mode: None,
         };
         Ok(factory)
     }
@@ -548,6 +561,7 @@ impl MimeFactory {
             sync_ids_to_delete: None,
             attach_selfavatar: false,
             webxdc_topic: None,
+            pre_message_mode: None,
         };
 
         Ok(res)
@@ -779,7 +793,10 @@ impl MimeFactory {
         headers.push(("Date", mail_builder::headers::raw::Raw::new(date).into()));
 
         let rfc724_mid = match &self.loaded {
-            Loaded::Message { msg, .. } => msg.rfc724_mid.clone(),
+            Loaded::Message { msg, .. } => match &self.pre_message_mode {
+                Some(PreMessageMode::PreMessage { .. }) => create_outgoing_rfc724_mid(),
+                _ => msg.rfc724_mid.clone(),
+            },
             Loaded::Mdn { .. } => create_outgoing_rfc724_mid(),
         };
         headers.push((
@@ -893,7 +910,7 @@ impl MimeFactory {
             ));
         }
 
-        let is_encrypted = self.encryption_pubkeys.is_some();
+        let is_encrypted = self.will_be_encrypted();
 
         // Add ephemeral timer for non-MDN messages.
         // For MDNs it does not matter because they are not visible
@@ -978,6 +995,22 @@ impl MimeFactory {
             "MIME-Version",
             mail_builder::headers::raw::Raw::new("1.0").into(),
         ));
+
+        if self.pre_message_mode == Some(PreMessageMode::FullMessage) {
+            unprotected_headers.push((
+                "Chat-Is-Full-Message",
+                mail_builder::headers::raw::Raw::new("1").into(),
+            ));
+        } else if let Some(PreMessageMode::PreMessage {
+            full_msg_rfc724_mid,
+        }) = self.pre_message_mode.clone()
+        {
+            protected_headers.push((
+                "Chat-Full-Message-ID",
+                mail_builder::headers::message_id::MessageId::new(full_msg_rfc724_mid).into(),
+            ));
+        }
+
         for header @ (original_header_name, _header_value) in &headers {
             let header_name = original_header_name.to_lowercase();
             if header_name == "message-id" {
@@ -1119,6 +1152,10 @@ impl MimeFactory {
                         for (addr, key) in &encryption_pubkeys {
                             let fingerprint = key.dc_fingerprint().hex();
                             let cmd = msg.param.get_cmd();
+                            if self.pre_message_mode == Some(PreMessageMode::FullMessage) {
+                                continue;
+                            }
+
                             let should_do_gossip = cmd == SystemMessage::MemberAddedToGroup
                                 || cmd == SystemMessage::SecurejoinMessage
                                 || multiple_recipients && {
@@ -1875,9 +1912,13 @@ impl MimeFactory {
 
         // add attachment part
         if msg.viewtype.has_file() {
+            if let Some(PreMessageMode::PreMessage { .. }) = self.pre_message_mode {
+                // TODO: generate thumbnail and attach it instead (if it makes sense)
+            } else {
                 let file_part = build_body_file(context, &msg).await?;
                 parts.push(file_part);
             }
+        }
 
         if let Some(msg_kml_part) = self.get_message_kml_part() {
             parts.push(msg_kml_part);
@@ -1921,6 +1962,8 @@ impl MimeFactory {
             }
         }
 
+        self.attach_selfavatar =
+            self.attach_selfavatar && self.pre_message_mode != Some(PreMessageMode::FullMessage);
         if self.attach_selfavatar {
             match context.get_config(Config::Selfavatar).await? {
                 Some(path) => match build_avatar_file(context, &path).await {
@@ -1990,6 +2033,20 @@ impl MimeFactory {
 
         Ok(message)
     }
+
+    pub fn will_be_encrypted(&self) -> bool {
+        self.encryption_pubkeys.is_some()
+    }
+
+    pub fn set_as_full_message(&mut self) {
+        self.pre_message_mode = Some(PreMessageMode::FullMessage);
+    }
+
+    pub fn set_as_pre_message_for(&mut self, full_message: &RenderedEmail) {
+        self.pre_message_mode = Some(PreMessageMode::PreMessage {
+            full_msg_rfc724_mid: full_message.rfc724_mid.clone(),
+        });
+    }
 }
 
 fn hidden_recipients() -> Address<'static> {

src/test_utils.rs

@@ -711,6 +711,32 @@ impl TestContext {
         })
     }
 
+    pub async fn get_smtp_rows_for_msg<'a>(&'a self, msg_id: MsgId) -> Vec<SentMessage<'a>> {
+        self.ctx
+            .sql
+            .query_map_vec(
+                "SELECT id, msg_id, mime, recipients FROM smtp WHERE msg_id=?",
+                (msg_id,),
+                |row| {
+                    let _id: MsgId = row.get(0)?;
+                    let msg_id: MsgId = row.get(1)?;
+                    let mime: String = row.get(2)?;
+                    let recipients: String = row.get(3)?;
+                    Ok((msg_id, mime, recipients))
+                },
+            )
+            .await
+            .unwrap()
+            .into_iter()
+            .map(|(msg_id, mime, recipients)| SentMessage {
+                payload: mime,
+                sender_msg_id: msg_id,
+                sender_context: &self.ctx,
+                recipients,
+            })
+            .collect()
+    }
+
     /// Retrieves a sent sync message from the db.
     ///
     /// This retrieves and removes a sync message which has been scheduled to send from the jobs