Hosted Exchange email users experince problems with incoming or outgoing email messages delivery, they cannot send or receive mail. Sent email messages are going out from outbox and may be seen in Sent folder in Outlook/OWA, however they are not being received by recipients.
Things to try1. Check if Exchange mailbox server where mailbox is located is up and running at all.
2. Use Exchange native tools to investigate mail flow, e.g. use the Get-MessageTrackingLog cmdlet in the Exchange Management Shell or Toobox > Mail flow tools in the Exchange Management Console.
3. Check and restart if needed the Microsoft Exchange Information Store service on the corresponding mailbox server where mailbox resides.
4. Log into Exchange Hub Transport server and check Exchange Submission queue. If you see abnormal amount of undelievered messages in the queue try to restart Microsoft Exchange Transport service.
5. If you see the error messages Primary target IP address responded with 421 4.2.1 Unable to connect or 451 4.4.0 DNS Query Failed in the Queue Viewer in the Exchange Management Console or in Exchange hub transport logs, then the problem may be related to the incorrect DNS configuration on the Exchange Hub Transport servers. Follow the Microsoft KB article Troubleshooting Mail Queues That Are Increasing on Edge Transport Servers to correctly configure DNS resolving on Exchange Hub Transport servers.
Check if the message was delivered after that.