[Bug 3171] New: wishlist: Add flag to mark recipient as unique to allow duplicate delivery

https://bugs.exim.org/show_bug.cgi?id=3171

            Bug ID: 3171
           Summary: wishlist: Add flag to mark recipient as unique to
                    allow duplicate delivery
           Product: Exim
           Version: N/A
          Hardware: All
                OS: All
            Status: NEW
          Severity: wishlist
          Priority: medium
         Component: Routing
          Assignee: [email protected]
          Reporter: [email protected]
                CC: [email protected]

This feature request is about adding a flag to allow duplicate delivery.

Rationale: Major online email providers automatically save a copy of every mail
sent to a 'Sent' folder. To mimic this behavior, the most straightforward
approach is using a redirect router marked as unseen, bcc-ing every submitted
mail to the sender, with some additional header to deliver it to a mailbox
named something like 'Sent'.
This process works mostly ok, though there are edge cases: if the user sends a
mail to itself, the duplicate delivery prevention mechanism will result in only
one mail present in either inbox or Sent.

Another possible approach is to use a local part suffix starting with + to mark
mails that should be placed in Sent. However, this results in a security risk:
If someone sends a mail to [email protected], this mail would end up in his
Sent mailbox instead of inbox.

I believe the most elegant solution is to have a flag that marks recipient to
be unique such that this delivery doesn't take part in the
duplication-prevention mechanism.

Currently I use a header named X-Save_to_mailbox, which is stripped from
incoming emails and added when redirected, and later processed by a sieve to
put the mail into mailbox Sent. It mostly works, except when I'm sending an
email to myself. Currently pipe delivery could circumvent
duplication-prevention, but that isn't feasible for me since I'm using LMTP to
save to a mdbox managed by dovecot.

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