Using Office 365 Business Premium only here... No On-Premises Exchange Server, all in-cloud.
I have a user who is not receiving internal emails from some other users.
I have run message trace: When I trace the sender's mailbox I can see the undelivered messages have a status of Pending (going back a few days) while all other emails say delivered. Details say they have been deferred (450 4.4.312 DNS query failed). When I trace the recipient's mailbox, I don't even see the messages, so I know they haven't hit that mailbox yet. Clearly the problem is happening upon send, not receive.
Strangely, this is not happening on all internal email accounts, as I can email the affected user and they get it. It is only happening with some internal senders. And it looks like only the one recipient is affected, though I can't confirm this, they are the...