1. Check for any 'Insufficient Storage' errors in the Exhange Server's Event Viewer.
2. See if any emails have been stuck in the Drafts folder for an unusually long time.
3. Create a new mailbox to check if the same issue is seen there.
4. Analyze the disk space on the server to identify any issues with the hard drive.
5. Try sending messages from Outlook Web Access to check for any connectivity issues with Exchange Server.
6. Try logging in with another account to check if the same error is seen there.
7. Check the user's mailbox size limits to make sure it hasn't exceeded the specified limits.
8. Rebuild and Reindex the user's mailbox.
9. Verify that clients are connecting to the correct exchange server.
10. Get in touch with Microsoft for further assistance in resolving the issue.