Outlook: Email sent to internal email ID, getting delivered to another ID

[BINODMAHARJAN2022]

Hello, 

 Today I have an interesting issue in email. From Outlook Desktop app, when email send to internal domain email id, it's getting delivered to another email ID that is gmail ID. 

In outlook sent item box, there is internal valid email address shown.

But in exchange online message trace, message is delivered to gmail ID address and that gmail address is receiving email as well. 

How do i trace, what might be the issue? I have checked those gmail address list in outlook contact list as well but nothing found.

[#2]

When you look at the (detailed) message trace, it will tell you why the message was delivered to another address. It's usually due to some form or forwarding, run Get-MessageTraceDetail to see what exactly happens.

[#3]

In message trace, there is no trace of sending email to internal email ID. There is only email address of gmail which showing delivered. I already checked forwarding and rules.
But in outlook desktop app sent item box, it is showing internal email ID.
My question is that, how this might happen? and trace it.