I have a distribution for my staff email@example.com. We have an outside company send this email address a newsletter. If I email this email list internally from the "all staff" everyone gets the email. When an email is sent to its external email address not all users receive the email.
Have checked one of the users in question, he is part of another list that is part of the main company list. Checked logs looks like it resolves most of the names when email is received to firstname.lastname@example.org but not all.
Any ideas helpful.