Hi Ieva,
Can you still see these missing sent mails if we check it from the webmail interface in a browser?
Which type of email account are you using? IMAP, POP3 or Exchange? After reinstalling Office, did it ask you to reconfigure your email account in Outlook?
Do you have any 3rd-party add-in installed? If any, we may try starting Outlook in safe mode and see whether this issue continues. To do this, close Outlook, press Win key + R to open the Run command, type
outlook /safe and press Enter.
Besides, we can also try starting Outlook with /cleanviews switch to reset your Outlook folder views and then see whether the issue still happens.
Regards,
Steve Fan
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