I’ve been testing with Domain Names that are not widely used by employees.
Sending from a Migrated user on O365 to a User on the On-Premise server I can send to some but not others. Test or non-test domain. The interesting thing is I’m seeing a Different Proxy Email Address for the users that I cannot send to. From my O365 user I’m sending to my Personal email at the On-Premise server and the O365 user is trying to send to: firstname.lastname@example.org. I’m not sure why it is @tenant.onmicrosoft.com and not the one with mail in it like this: @tenant.mail.onmicrosoft.com
What is the Difference between:
I don't have any user setup with the proxy address of: email@example.com
I do have a address policy defined to have all users have a firstname.lastname@example.org
I see tenant.onmicrosoft.com in the Office 365 Portal/Setup/Domains. The Domain tenant.mail.onmicrosoft.com is not listed there.