Does the problem happen on the client side or on the web side?
Will this be the case for all new emails? Or are there specific messages (such as specific recipients or containing links)? Please create a new email for other recipients to test.
Besides, you can use the following troubleshooting in this official document: I can't send or receive messages in Outlook
If issue persist, please do let me know and I'll do my best on continuing to help you.
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