Hi Douglas,
Thank you for your effort.
Based on the NDR, it seems the recipient account have problem.
May I have more information to narrow down the issue as follows:
1. Where is the recipient located which have issue for this DNR?(on-premise server? or Cloud server?)
2. Have you tried to use other account sent mail to this recipient and got same issue?
3. When did this issue take place? (this is very important please provide distinct information)
Thanks,
Alex Du