An external email address sent an email to three internal users, two of them received the email but one didn't.
And I tracked the mail flow in EMC, it showed that email delivered to those two users but no that one's record. And the return-path was the one's address who could not receive the email. Is it related to this issue? Normally, the return-path should be the sender's address but not the recepient's.
But the second time the same email was sent, three of them could receive. And the return-path normally showed the sender's address.
What's the root cause? I don't want it happen again in the future.