Is there a way to globally route incoming secondary domain mail to a primary domain for all users without having to list every possible combination of account alias and domain alias in each users AD proxyAddresses field?
For example users expect to be able to receive mail addressed to firstnamelastname, firstname.lastname, and in some instances firstname @primarydomain.com and the same @secondarydomain1.com, @secondarydomain2.com, etc.
I don't believe this can be done in Exchange Online due the limitations of the transport rules. It can be done if you route mail through a local Exchange setup with an Edge server however.
With this configuration you can create a rewrite rule with PowerShell using New-AddressReWriteEntry command.
Other 3rd party hosted services may offer this as well. I believe Barracuda has settings that allow for rewriting as long as you validated the domains in question.
