We're on Office 365/Exchange Online. We just completed a domain move from domain-old.edu to domain-new.edu. The .EDU TLD exclusive registrar (EDUCAUSE) is pretty strict about how long you can have two .edu domains - and we have another 5 months from here. The new domain is set up in Office 365, and everyone's primary SMTP is username@domain-new.edu, with a secondary of username@domain-old.edu. They're happily receiving mail for both addresses right now.
My dream is to be able to apply a transport rule that stamps a disclaimer on inbound messages sent to username@domain-old.edu that says something along the lines "This user used your @domain-old.edu address - please remind them to update their records, as @domain-old will stop receiving mail on October 15th, 2020" or so.
I've tried a few rules so far - recipient domain is @domain-old.edu, header To includes @domain-old.edu, but no luck yet. Any ideas on how to accomplish this?
I do already have 1 disclaimer rule in place to alert a user to "External Messages" for SPAM/Phishing Purposes.
Thank you in advance!