Here at my organization we have an email list/group and web have several emails wich include email from our domail address e some external people who uses gmail etc.
Onde particular user, we created an AD user name and set an Gmail email address as a contact.
The problem is that this particular user dont receive the emails sent by the list. To make a troubleshoot, we set another email address for the same user and on the new email address he receives the email correctly.
We dont know what is the issue, we double checked the email address and its correct, but for some reason, just on this particular email address nothing works.