what do you use for sending the vacation message? smeserver-vacation?
if yes this would need for the script to be aware of what are the allowed pseudonym. for that it will need some recoding.
The easy way of just using the To field would not work as you moght be as bcc or cc and this might be a redirection of email where your server is not authorized to send using this domain (eg forward adresses).
i suggest you to post this nfr against the contrib in bugzilla