Auto-reply users that subdomain no longer used
I have a subdomain fax.example.com which users have sent emails to for forwarding to a fax server, e.g. email@example.com (123456 is a fax number example).
The fax server is now being replaced with a cloud based solution and users will instead send emails to firstname.lastname@example.org (123456 is a fax number example)..
My question is if there is a way to auto-reply to users' emails at fax.example.com that they should send emails to the new email address email@example.com (123456 is a fax number example) during the transition period.
EDIT: Clearly my description above including the use of e.g. to provide an example fax-email address has been misinterpreted. There are no accounts under the subdomain fax.example.com. the "123456" in my example is a fax number.