Outlook Sending reported error (0x8004060C)

Task 'Microsoft Exchange Server - Sending' reported error (0x8004060C): 'The message store has reached its maximum size. To reduce the amount of data in this message store, select some items that you no longer need, an permanently (SHIFT + DEL) delete them.'

Check the "Prohibit send at" quota for the mailbox that is experiencing this issue.