Simple Multi-User Out-Of-Office Responder

We have a small e-mail server running on Debian GNU/Linux. All users are local (in /etc/passwd), have mail delivered via procmail to mailboxes in /var/mail and can access it with POP3, IMAP or squirrel-mail webmail. Pretty standard, simple setup which works quite well. The only thing missing was an “out of office responder”.