You want to inform external senders the accounts have been relocated.
Use relocated_maps for the Zimbra MTA
touch /opt/zimbra/conf/relocated postmap /opt/zimbra/conf/relocated postconf -e relocated_maps=lmdb:/opt/zimbra/conf/relocated zmmtactl restart postconf | grep ^relocated_maps relocated_maps = lmdb:/opt/zimbra/conf/relocated <<== you should see this
The format to use is simple, the old company address first, followed by the new address on the same line.
Anytime /opt/zimbra/conf/relocated is updated, be sure to run the postmap utility against it.
Now, anytime someone sends firstname.lastname@example.org a new message, they will get a bounce message back.
Subject: Undelivered Mail Returned to Sender This is the mail system at host zimbra.example.com. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the attached returned message. The mail system <email@example.com>: User has moved to firstname.lastname@example.org
More articles written by me, https://wiki.zimbra.com/wiki/King0770-Notes