In the meantime (or if supporting ":fcc" is not planned), is it possible to achieve the same result with the existing Sieve extensions? (I can imagine a way to do this at the MTA level by checking the presence of the "Auto-Submitted:" header, but did not test it.)
I can't think of any means to do that within the Sieve language.
I'll try to find a way to do it at the MTA level, but it seems more complicated than what I initially thought.
For the record, it seems that this can't be done with Postfix, at least not in a simple way. One option would be to keep a copy of all emails with 'always_bcc', and to forward autoreplied emails to the individual user mailboxes. Another option would be to write a specific milter, but this is beyond my abilities.
Will ":fcc" be supported in the future?
I'd say yes, but it is not simple to implement, nor is it high up in our list of priorities.
Perhaps ":fcc" is not simple to implement, but I personally would already be happy with a ":bcc" option, which would send a blind-carbon copy of the message to the given email address. Is this not easier to implement?
Gregory