[Feature Request]: Add SMTP listener and mapping support

[tiredofit]

Feature Description

Allowing incoming emails to this SMTP server by means of an upstream server.

This eliminates gaps when using IMAP and POP3 where the polling interval could be more than the time that messages have been received into the mailbox and been deleted or modified manually or with users filters.
A popular tool piler uses something similar to this request.

Proposed Solution

Postfix and other upstream mailservers can utilize something similar to always_bcc sender_bcc_maps or recipient_bcc_maps, which could map user@realdomain.com -> user@mailarchiver.realdomain.com, or be able to determine the to, cc, bcc from the headers via a global bcc email address and route the messages accordingly to users either auto provisioned or manual users.

This server shouldn't obviously send mail outbound and should fail silently should there be NDRs or loops. Bonus would be ability to add listening controls as to what interface, port to listen on, and TLS support.