1. Is there any third party gateway on user 2’s tenant? Is User1 or their domain whitelisted? Or if they are using EOP again are they whitelisted? 2. Have you done a track and trace in the tenants to see if anything was sent/received at the time of the offline message?
1. No third party gateway on User 2's tenant (or User 1's for that matter).
2. No explicit whitelisting exists between these two domains at any level.
3. Neither side is using EOP.
4. I have not done a "track and trace" on either end - can you tell me where I'd do that?
For the heck of it, I tried the exact same scenario with a different tenant (replace "User 2" with another user on a different tenant configured pretty much identically as far as your questions so far are concerned, and also, seemingly no email sent/received.
It may make no difference, but I routinely send email between all these users without issue.