First try

Cargo.lock

@@ -200,8 +200,7 @@ dependencies = [
 [[package]]
 name = "async-smtp"
 version = "0.3.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a9720181a7d56bf3b4d0cfdcb6353df975125996bdd2958b4e639c8317fcaa68"
+source = "git+https://github.com/async-email/async-smtp?branch=smtptimeout#74f489f29f8cbd2650d69119e63bf9e35e36dc85"
 dependencies = [
  "async-native-tls",
  "async-std",

Cargo.toml

@@ -20,7 +20,7 @@ smallvec = "1.0.0"
 surf = { version = "2.0.0-alpha.4", default-features = false, features = ["h1-client"] }
 num-derive = "0.3.0"
 num-traits = "0.2.6"
-async-smtp = { version = "0.3" }
+async-smtp = { git = "https://github.com/async-email/async-smtp", branch = "smtptimeout" }
 email = { git = "https://github.com/deltachat/rust-email", branch = "master" }
 lettre_email = { git = "https://github.com/deltachat/lettre", branch = "master" }
 async-imap = "0.3.1"

src/smtp/send.rs

@@ -5,6 +5,7 @@ use async_smtp::*;
 
 use crate::context::Context;
 use crate::events::Event;
+use std::time::Duration;
 
 pub type Result<T> = std::result::Result<T, Error>;
 
@@ -30,7 +31,7 @@ impl Smtp {
         message: Vec<u8>,
         job_id: u32,
     ) -> Result<()> {
-        let message_len = message.len();
+        let message_len_bytes = message.len();
 
         let recipients_display = recipients
             .iter()
@@ -47,11 +48,16 @@ impl Smtp {
         );
 
         if let Some(ref mut transport) = self.transport {
-            transport.send(mail).await.map_err(Error::SendError)?;
+            // The timeout is 1min + 3min per MB.
+            let timeout = 60 + (180 * message_len_bytes / 1_000_000) as u64;
+            transport
+                .send_with_timeout(mail, Some(&Duration::from_secs(timeout)))
+                .await
+                .map_err(Error::SendError)?;
 
             context.emit_event(Event::SmtpMessageSent(format!(
                 "Message len={} was smtp-sent to {}",
-                message_len, recipients_display
+                message_len_bytes, recipients_display
             )));
             self.last_success = Some(std::time::SystemTime::now());