If some are getting external email and others are not, there can be any number of reasons for the behavior. However, there should be some kind of non-delivery report generated.
!) The mail is arriving but being mishandled by Exchange. You may try momentarily stopping your mail delivery service in coordination with an external email being sent to see whether the queue holds the message. Then, you can dissect addressing to see whether it is directed as you intend.
2) A rule has been set to either not receive messages from certain recipients or has been set to route such messages to a different recipient.
3) The email as advertised does not match the recipient’s email. They are giving out the wrong address so the sender copies and pastes the wrong address.
4) The email is too large. Filters may prohibit receiving email over some limitation set by default or by the Exchange administrator.
But it’s been four years so I figure you should tell all of us what you ended up finding.