Hi there,
The reason why you get the error is that Outlook is not authenticating your account on the server while sending it, so the outgoing mail server rejects the message.
You can try these steps and see if they are helpful
Check the Internet Connection.
Check the settings for the Email Account in Outlook.
Delete email messages if they look suspicious.
Check the antivirus application & disable it.
Check firewall configurations.
Start Outlook in Safe Mode.
Restart the PC after these steps and then try using Outlook again.
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