Email getting quarantine if sent to email alias.

Hello All,

Need help with one weird issue. 

Even though email is sent to mailto:xyz@domain.com it actually resolves to mailto:xyz@domain.onmicrosoft.comand then gets quarantine where there is no detail for quarantine if I run message trace. 

The sender is sending email to mailto:xyz@domain.com but in message trace it shows that email was sent to mailto:xyz@domain.onmicrosoft.com.

I have done some testing such as message trace , but didn't find anything. Any idea what could be wrong ! FYI I am using hybrid environment.