Hello, We got an Exchange 2013 as a mailserver. Actually we are having problems receiveing mails but just in a case.
We have deleted a mailboxuser, so I realized that if the deleted user is in CC or TO fields and other user are included in this fields too, the mail is rejected by our Exchange and nobody receives the mail. Users that exists not receiving the mail neither. The NDR is 550 5.1.1 User unknown.
If this deleted user is not included on the email the mails is received well.
It happens with all deleted user or not existant users.