Not all external contacts receiving distribution group email

[VinceMHAW]

We have a hybrid setup.   We have setup a distribution group with six external contacts and one internal email address.  When an email is sent to the group, three of the external addressees do not receive the email.  I have reviewed the property sheets for the external contacts in the AD, comparing them sheet-by-sheet, and they are identical. We have sent email directly to their individual address to be sure they are accurate, and they receive the individual email.

What might cause some not to receive the email?

[#2 · Accepted solution]

@VinceMHAW You should do a message trace to see if it makes things more clear 

https://protection.office.com/messagetrace

[#3]

@ChristianBergstromThanks, that did it.

Well, it did it insofar as it showed that the test email had in fact been delivered to each member of the distribution group.  Now I'll have to help them find them.  But I'd been reviewing the setup of the group repeatedly, trying to figure out what was wrong, so it's good to have that question removed.