Messages for a specific user are stuck in the Outbox

Hi @CranbinSun

Welcome to our forum!

For better research, please provide more details about this case:

What is your account type (POP/IMAP/Exchange)?

Do you send a message with a large attachment?

Please try to send this email to this user in OWA to check if it can be sent successfully.

If the problem only happens in Outlook desktop client, try to fix it by following the methods provided in this article.

I can't send or receive messages in Outlook - Microsoft Support.

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