Allow redirect to sender

[nmxhc]

Hi,

I would like to build an equivalent of virtual alias files using sieve scripts and thus need to potentially redirect a mail to its sender. Imagine e.g. getting a request to [email protected] which redirects to a bunch of private mails. If I now answer from one of these private mails, I usually put [email protected] in CC and receive a copy of my mail.

This is prevented by

sieve/src/runtime/actions/action_redirect.rs

Line 44 in 0e24d13

// Try to avoid forwarding loops

Relevant portions of the Sieve RFC 5228:

Implementations MUST take measures to implement loop control,
possibly including adding headers to the message or counting Received
headers as specified in section 6.2 of [SMTP]. If an implementation
detects a loop, it causes an error.

And Section 6.2 says

Simple counting of the number of "Received:" headers in a message has
proven to be an effective, although rarely optimal, method of
detecting loops in mail systems. SMTP servers using this technique
SHOULD use a large rejection threshold, normally at least 100
Received entries. Whatever mechanisms are used, servers MUST contain
provisions for detecting and stopping trivial loops.

Even with the last sentence, I think that the current approach is overly restrictive and counting Received headers would be a good approach.

Alternatively, this behavior could be limited to trusted scripts or behind a flag.