Not to sure if I'm going to be flying over peoples heads here or if I can get assistance with this here... but you won't know until you try... so I'm going to try!
The company I work with runs our own website server (vm3) and we've got 2 companies that have their websites on there (lets call these c1 and c2)
for the longest time, both could email eachother without any issues (they were local accounts so why would there be an issue right?)
Recently C2 decided they wanted to migrate their email hosting to Office 365, so we did that for them, pointed the MX records there, verified email was flowing properly, & disabled 'local host' for email on vm3.
Now here's the issue
C1 goes to email an account that was previously located on vm3 (for example Caveman@c2.ca) they get the following error