Hi,
What's the type of you account configured on your Outlook desktop client? Exchange?
Can you send mail normally?
Given this situation, just as Marcus said, if you are using cached mode, please try start your Outlook in online mode and see if there is any difference.(File > Account Settings > Double click your acccount > Uncheck "Use Cached Exchang Mode
to download email to an Outlook data file".)

Besides, your send/receive file may be corrupted, please close your Outlook client > Press Win+R to open Run > Type "%USERPROFILE%\AppData\Roaming\Microsoft\Outlook" and press Enter > Rename .srs files to .old > Restart your Outlook
desktop client and test.
If the issue continues, please try creating and using a new Outlook profile via Control Panel > Mail > Show profile and see if there is any difference.
Hope this can be helpful.
Regards,
Aidan Wang
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