Hi Lee,
We appreciate the update and we are sorry for the inconvenience.
Based on the description and my experience, the issue can be caused by the following reason:
1. If these 3 accounts are in the same Outlook mail profile, the other 2 connections may affect the Office 365 account.
2. Some third-party applications or add-ins may also cause the issue.
Given this situation, could refer to the following points to help us to find the root cause?
1. To check if the issue is caused by the first possilbe reason, please only add the affected Office 365 account in the new profile.
2. If it is possible, please add the affacted Office 365 account in a fresh-installed computer ( don't add any add-ins, and only add the affacted Office 365 account). This can also help us to narrow down the root cause. You time and efforts are highly appreciated.
To help reproduce the issue and further troubleshooting, please also provide the following detailed information.
1. could you tell me the version of the Outlook version?
2. When you mentioned "Microsoft exchange", which Office 365 plan you are using, such as Office 365 Small Business or Office 365 Enterprise.
Best Regards,
Anna Shi