Hi comdini,
What's the type of the affected account?
What if pressing F9 (Send and Receive) when the issue occurs?
When the issue occurs, please have a look at the connection status of the mailbox via the status bar at the lower-right corner of the window and see if it shows as "Connected" or "Connected to: Microsoft Exchange".
Besides, it is suggested to check if the affected users have installed any third-party add-ins in common which may lead to this issue.
Any updates, feel free to post back.
Regards,
Yuki Sun
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