If you just need the number of messages, you can use the Get-MailFlowStatusReport cmdlet
Mail Flow.
Microsoft Q & A
381
Reputation points
How to get the count of emails sent to external (outside) from all the accepted domains in office 365 tenant. I need to work on DKIM implementation and correct me if I am wrong , only emails sent to external will have DKIM sign in. I need to check which domain is sending most number of email outside and required DKIM implementation first.