Hi
I have an interesting ‘opportunity’ with a customer, where they are looking to provide email (Exchange Online) to retirees. So with that in mind we have setup the domain retiredcompany.com on Office 365. We have also setup redirects to send email sent to user@retired.company.com to office 365 mailboxes. This all works fine. email sent to user@retired.company.com gets routed to the appropriate inbox.
Now to complicate matters, when a user retires they will be moved to a cloud mailbox. A forwarder will be placed on Exchange on-premises to forward user@company.com mail to the users retiredcompany.com mailbox. Again all works fine.
The problem is that the customer wants to block the cloud user from sending messages to their user@company.com address. This will be an issue when the user "replies all" to messages that contain their old user@company.com address.
Now I know I can use transport rules in Exchange Online, but I can't figure out a way to block this scenario without having to build a rule for each retiree, which will not scale. I need a transport rule that will compare the username portion (left of @) of the senders address to the username portion (left of @) of any recipients address and block the message from being sent to that recipient when they match.
Notes:
We have structured the username to match the on-premises usernames, so the match should work.
The customer is fine with blocking messages if the users send messages to their old email addresses, so it's ok if we will intentionally be blocking some good messages.
The customer doesn't want the users to receive messages that they send unknowingly to an old email address that is being forwarded.