Hi @Anonymous
In order to better study this issue, please provide more details:
Are you the only user in your organization experiencing this issue?
What is your account type (Exchange/POP/IMAP)?
Did you add other accounts in the Outlook client in the same profile as your Office 365 account? This issue occurs when a Google Apps Sync for Outlook account and an Exchange Server account are configured in the same Outlook profile. More information, see: The operation failed error when you send email messages in Outlook - Outlook | Microsoft Learn.
Here are some suggestions for your reference:
- Switch to Online mode and then click "Empty auto-complete list" again. Click File->Options->Mail->Send Messages, and then click the Empty Auto-Complete List button.
- Open outlook in safe mode to check if there any add-ins cause this issue.
- Repair Office application.
A similar thread on this issue for your reference: Outlook 2016 Cannot resolve recipient.
Hope this helps!
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