GMail's submission server saves all the submitted mails to the sender's Sent
mailbox. After this most IMAP clients with their default configuration will
again save the same mail to the Sent mailbox. I'm not sure if GMail removes one
of them as a duplicate or not. I've been considering doing a similar feature to
Dovecot (it will most likely have a proxying submission server in near future),
but it would be nice to tell the SMTP clients that this auto-saving is
happening, so they could skip the IMAP APPEND part with default configuration.
Of course there are the LEMONADE extensions that are supposed to solve this
same issue, and Dovecot will support those as well, but I think it's going to
take a while for clients to actually implement those. There's likely much
better chance of them implementing a small check for "oh, the mail was already
saved, I'll just skip the IMAP APPEND." Even if any clients themselves didn't
implement this, it would make it possible to implement a simple SMTP+IMAP proxy
locally that catches the flag and supresses the APPEND before it reaches the
network.
So, any thoughts on creating an RFC for this functionality? I'm not familiar
enough with SMTP yet to know what would be the best design for that
notification. Maybe a new extended status code? Maybe change some of the DATA
reply's text part to have a special meaning? A simple flag is enough I think,
because it's reasonable to require that both the server and client supports
IMAP SPECIAL-USE to find the \Sent flagged mailbox. Returning IMAP
UIDVALIDITY+UID could help IMAP clients a bit more, but that's maybe a bit too
ugly (and not always possible).
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