Shortly we will be moving to a new name and thus we are ready to set a new domain (abc.com) as 'default' on our 365 tenant. In the mean time we would like to keep the current domain-adresses (xyz.com) 'active' for a period of time in order to miss no mails, but we would also like to warn people sending to the xyz.com- addresses that they are using an address that will soon be obsolete.
On the 365 exchange settings I don't see how this can be done. (But probably I'm overlooking something) Any hints on a solution?
Such scenarios are usually addressed by adding an Automatic reply to each and evert mailbox you expect people will still be sending messages too. A centralized solution is also possible, but you will loose the ability to alert the sender for the "best match" new address.
Another thing to have in mind is that Automatic replies only trigger once per sender. If you want this warning to be displayed on each mail received, you can use Inbox rule instead.