Hi @Pero ,
Drop:
In your case, I think it's the second reason.
It could be deleted by the transport rules or some anti spam agents.
You could use the following command to check the details about the Drop Event:
Get-MessageTrackingLog -EventID Drop -MessageSubject "Subject" -Sender ******@domain.com -recipients ******@domain.com | FL
If you have some transport rules or enabled the anti spam protection( like third-party security software), you may check them first.
Also you can provide the result of the above cmdlet with your information covered.
Best regards,
Lou
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