I have noticed in mailaccess.log when a mail arrives to a user with an autoanswer filter active and configured, the part of the system that is responsible for delivering to the user's mailbox folder, which i think is called DOVECOT, puts a log saying
"discarding vacation response for implicitly delivered message; no known (envelope) recipient address found in message headers (recipient=<user@hostname>, and additional `:addresses' are specified)"
and that gave me a hint.
The problem is that DOVECOT does not uses the user's mail address.
After looking a way to correct this I found a line in master.cf, wich is located in /etc/postfix, that says
dovecot unix - n n - 25 pipe
flags=DRhu user=_dovecot:mail argv=/usr/libexec/dovecot/dovecot-lda -d ${user}
# Use these settings to enable +addressing (user+mailbox@example.com)
# flags=DRhu user=_dovecot:mail argv=/usr/libexec/dovecot/dovecot-lda -d ${user} -a ${recipient} -m ${extension}
I've added at the end of the flags line what is suggested
-a ${recipient} -m ${extension}
and reloaded postfix master.cf
After that, vacation messages started to be sent.
Hope this helps.