Hi John,
The issue could be caused by the server information for the connected accounts has been updated on the server side, however it’s not detected in Office 365. To solve the issue, I suggest you can enter the detail server information for the connected accounts and try again. To do that, please try the steps below:
- Find the POP/IMAP settings information on the connected accounts’ server side.
- Login in OWA (Outlook Web App), find the URL like https://server.outlook.com (without the others), change it to https://server.outlook.com/ecp/PersonalSettings/NewPopSubscription.aspx or https://server.outlook.com/ecp/PersonalSettings/NewImapSubscription.aspx .
- Then enter the detail information for the connected account and give it a try.
For more details, please refer to: Enter POP or IMAP server information when connecting to another account.