Flag email to specific domain

[pbaldy]

We are in the process of transitioning to a new domain for email. Users can currently receive email at either (paul@abc.com or paul@xyz.com). I'd like to find some way of alerting our users when email is sent to the old domain, so they can contact the sender. Displaying the "To" field in Outlook doesn't work; it shows the same thing either way. I tried a rule that affected the category if "xyz" was in the recipient's address, but couldn't get that to work. Email comes through Exchange server if that's relevant.

Thanks for any thoughts,

Paul