Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.
TRUSTED BY
Check to see if mailbox size limitations have been reached.
Is this happening to one user or many?
If happening to many or all, make sure all Exchange services are started.
What do the queues look like?
Can the user send/receive via OWA?