Hi, Did you manage to solve your problem.
I'm getting the same mail message sent at least once a day. Cleared out messages. Checked my mailbox size which is nowhere near what the mail message says.
Just wondered if you'd found a solution.
I keep getting the same message from MicrosoftExchange 'Your mailbox is full. 977MB 972mb
your mailbox can no longer send messages. etc.
Sorry to hear you are having those issues.
In order to resolve that you would need to clear mailbox storage. You would have to do this online via your webmail.
To access that you would need to go to portal.office365.com and log in there with your email. From there you can delete emails as you see fit. Please ensure that you also delete them from the "Deleted Items" folder. From my experience it is usually Sent Items that clog up a mailbox, so double check your sent items folder to see if that is the case.
All the best with this one!