This is driving me crazy. I have two scenarios where users are getting 5.2.1 NDRs.
User1 gets the 5.2.1 NDR when sending any mail to user2. User1 can send mail to anyone else without issue. User2 receives mail from others without issue. The issue is not consistent. Sometimes mail from user1 to user2 is delivered without issue.
This user getting the 5.2.1 NDR on any meeting requests sent or accepted to several users Regular mail works fine, and user has no issues receiving mail.
I have already checked the obvious:
- No MBXs are disabled
- All associated accounts has valid SIDs
- Neither of the two scenarios involve sending mail to a public folder
- No phantom delegates anywhere
- Messages being sent are not too large
I am at a loss. Thoughts?