Define all Email aliases in dovecot autoresponder
Dovecot triggers only a autoresponder when the email is addressed to the mailbox address and not when it bis addressed to a email alias that points to this mailbox. The message that is logged in that case is:
Oct 20 20:21:30 ispconfig-mx dovecot: deliver(abc@domain.tld): sieve: msgid=4BF329.40309@domain.tld: discarding vacation response for message implicitly delivered to abc@domain.tld
http://www.dovecot.org/list/dovecot/2010-February/047036.html