So I want to put this out there, because I googled the crap out to this hand got zero information.
When doing an exchange message trace, it was used to see incoming email. going to have to explore this more, and find out what it really can do.
PS C:\Windows\system32>
PS C:\Windows\system32> Get-PerimeterMessageTrace -Recipient USER@SOMEPLACE.com
RunspaceId : 41c7b470-4aa1-41af-b3af-a617744583e9
Agent : PFA
Event : OnEndOfHeaders
Action : RejectMessage
SmtpResponse : 451 4.7.500 Server busy. Please try again later from [xxx.xxx.xxx.xxx]. (AS77712315)
Recipient : USER@SOMEPLACE.com
Sender : NOTHTEUSER@SOMEPLACE.com
Timestamp : 2019-02-27T11:06:00.000Z
ReferenceId : ab39b4a8-3ea2-4c75-b138-9e1383e4cad8
Identity : 4f07bbce-a87f-4c76-8739-f751c56433a2
IsValid : True
ObjectState : New