I have a small distribution group email address setup in my exchange server that has three internal employee as members and one "contact" record member. The contact record is a contractor employee who needs to be copied on these emails (but obviously has a different email address than employees of our company). On certain occasions, especially with one particular customer, when an email is sent to the distribution list, the contractor complains that he never received the email, even though the internal employees do get the email. When I go to message tracking on the exchange server, I see that there is a NDR generated (though it does not indicated why or to who). When I look in the exchange servers Event Log (Application), I can see that when this email comes in, there is an error generator saying:
Event ID: 3030
A non-delivery report with a status code of 5.6.1 was generated for recipient rfc822;firstname.lastname@example.org (Message-ID <5226F1B3.email@example.com>)
The NDR is only occasionally generated with certain customers (always it seems with this particular customer) for the contractor's email address. Yet, the email is successfully delivered to our internal employees all the time. Any idea on why this contractor's email address is sometimes rejected? If have sent numerous test emails from my personal yahoo email account to this distribution list email, and it always successfully reaches the contractor employee. Even other customers are successful, but a few are not.