Hi,
Welcome to the Microsoft forum.
According to your description , may I please know if this issue only occurs in Outlook? If you use OWA, will it occur?
1.
If the issue does not occur in OWA, we could consider that the mailbox is working fine and it may be something wrong with Outlook, you can try to do the following:
- Open Outlook in safe mode and troubleshoot add-ins,more details you can refer to : Fix Microsoft Outlook Error 0x80040115 in Windows 11/10 (thewindowsclub.com)
- Try to use SARA to fix the problem,download it in this link: Download Microsoft Support and Recovery Assistant from Official Microsoft Download Center
If the issue also occurs in OWA, you may need to check the status of your E3 license. For example, if the license has expired, you will not be able to use all the features under the license including access to Exchange mailboxes.
Access to the Microsoft 365 admin Center,select Billing->Your products:
Check if the status of the E3 license has expired.
I hope this helps.Please feel free to contact me for any updates.