Hi Prashant,
According to your description, do you mean the mails were still sending, but also left a copy(0 bytes) in the outbox?
Or do you mean the mails were stuck in the outbox and can not be sent at all?
Does this issue only affect the IMAP account?
To help narrow down the issue, please try starting Outlook in safe mode(Press Win + R, type "outlook /safe", press Enter.) and check if the issue still persists.
Besides, you may also try to open up the account settings, go to Change > Advanced, add "inbox" under root path and see if there will be any improvement.
Any updates, please feel free to post back.
Best regards,
Yuki Sun
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