Hello Edd,
Thank you for reaching out.
Kindly note that there is an incident that was posted to the Service Health with ID EX742767 which might be related to this behavior. The user experience of this incident is: Legacy Outlook for Mac users may be unable to connect to their Exchange Online mailboxes.
Title: Legacy Outlook for Mac users may be unable to connect to their Exchange Online mailboxes
User impact: Legacy Outlook for Mac users may be unable to connect to their Exchange Online mailboxes.
Current status: The fix is continuing to deploy to the affected environment. We're monitoring its progress and anticipate it may be finished saturating by our next scheduled communication update. We'll confirm its completion or reassess the estimated time for resolution at that time.
Scope of impact: Any user of the legacy Outlook for Mac hosted on the affected infrastructure may be impacted.
Root cause: A recent service maintenance change introduced a code issue that inhibits communication between legacy Outlook for Mac users and the Exchange Online service, which is resulting in the impact.
For affected customers to easily identify the information here, this thread may be closed. I will also post back once this issue is resolved in Service Health.
Thank you for your kind understanding.
Best Regards,
Ashraf | Microsoft Community Moderator