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/constants.rs

@@ -209,6 +209,11 @@ pub const WORSE_IMAGE_SIZE: u32 = 640;
 // this value can be increased if the folder configuration is changed and must be redone on next program start
 pub(crate) const DC_FOLDERS_CONFIGURED_VERSION: i32 = 4;
 
+// If more recipients are needed in SMTP's `RCPT TO:` header, the recipient list is split into
+// chunks. This does not affect MIME's `To:` header. Can be overwritten by setting
+// `max_smtp_rcpt_to` in the provider db.
+pub(crate) const DEFAULT_MAX_SMTP_RCPT_TO: usize = 50;
+
 #[cfg(test)]
 mod tests {
     use num_traits::FromPrimitive;