fix: Split SMTP jobs already in chat::create_send_msg_jobs() (#5115)

a27e84ad89 "fix: Delete received outgoing messages from SMTP queue"
can break sending messages sent as several SMTP messages because they have a lot of recipients:
`pub(crate) const DEFAULT_MAX_SMTP_RCPT_TO: usize = 50;`

We should not cancel sending if it is such a message and we received BCC-self because it does not
mean the other part was sent successfully. For this, split such messages into separate jobs in the
`smtp` table so that only a job containing BCC-self is canceled from `receive_imf_inner()`. Although
this doesn't solve the initial problem with timed-out SMTP requests for such messages completely,
this enables fine-grained SMTP retries so we don't need to resend all SMTP messages if only some of
them failed to be sent.

src/smtp.rs

@@ -595,7 +595,13 @@ pub(crate) async fn send_msg_to_smtp(
     match status {
         SendResult::Retry => Err(format_err!("Retry")),
         SendResult::Success => {
-            msg_id.set_delivered(context).await?;
+            if !context
+                .sql
+                .exists("SELECT COUNT(*) FROM smtp WHERE msg_id=?", (msg_id,))
+                .await?
+            {
+                msg_id.set_delivered(context).await?;
+            }
             Ok(())
         }
         SendResult::Failure(err) => Err(format_err!("{}", err)),