Exchange 2016 Distribution Group doesn't work for external email

I have a client with a new Exchange 2016 server.  We setup a Distribution Group.  Email sent internally to the Distribution Group are forwarded as expected.  Email sent externally to the Distribution Group are never sent.  Distribution Group is working for internal email addresses but not external email addresses.

I reviewed Delivery Management for the Distribution Group.  "Senders inside and outside of my organization" is selected.  There are no restrictions setup - "All senders can send messages to this group".  Nothing is setup in Message Approval - no boxes are checked.  What am I missing?  Is there a global setting somewhere that's overriding the Distribution Group settings?
David XFSystems AdministratorAsked:
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FOXActive Directory/Exchange EngineerCommented:
Are you sure your external users are emailing the correct smtp address for that distribution group.  When an external sender sends an email what error do they get?
David XFSystems AdministratorAuthor Commented:
Positive the external senders are using the correct address.  Copy and pasted into Outlook.
External senders are not receiving any errors or alerts.  Email is just not sent.  Exchange is acting as if "Only senders inside my organization" is selected, but it's not.
FOXActive Directory/Exchange EngineerCommented:
The distribution list was set up as a Universal Group, correct?
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David XFSystems AdministratorAuthor Commented:
Yes.  It's setup in AD as a Universal Distribution Group.
FOXActive Directory/Exchange EngineerCommented:
Send another email in to the DL and check your message tracking simultaneously.
David XFSystems AdministratorAuthor Commented:
I may need some help with the message tracking.  I'm used to using Message Tracking and Tracking Log Explorer in Exchange 2010.  I'm not seeing either of those in Exchange 2016.  Are they hidden somewhere?
FOXActive Directory/Exchange EngineerCommented:
In Exchange Admin go to Delivery Reports

ref link:

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☁ François Peroux ☁Infrastructure Consultant - Cloud Solutions | Consultant Infrastructure - Solutions CloudCommented:

You can find tracking in the Delivery Report EAC console, just under Mail Flow > Delivery Reports.
David XFSystems AdministratorAuthor Commented:
Thanks for the help.  I'm being pulled offsite for the rest of the day, so I'll continue working on this issue tomorrow morning.
FOXActive Directory/Exchange EngineerCommented:
David XFSystems AdministratorAuthor Commented:
First, the Delivery Reports in the EAC are worthless.  I just taught myself how to pull logs using get-messagetrackinglog in the management shell and was able to get information that actually helps.

It seems the problem has nothing to do with external email sending to a distribution group.  The problem is that the distribution group contains external email addresses.  If an external email sends to the distribution group, internal users that are part of the distribution group receive the email, external users that are part of the distribution group do not receive the email.

It looks like an issue with our Exchange and Barracuda Spam Filter?  (We route all email through Barracuda).

ConnectorId             : Barracuda Send Connector
Source                  : SMTP
EventId                 : DEFER
Recipients              : {}
RecipientStatus         : {451 domain not found (not relaying for}
FOXActive Directory/Exchange EngineerCommented:
Set up those external email addresses as contacts of your Exchange and them to the distribution group and see if the external emails addresses get the emails
David XFSystems AdministratorAuthor Commented:
Already done.  The external users in the distribution group are already setup as Contacts in Exchange.
☁ François Peroux ☁Infrastructure Consultant - Cloud Solutions | Consultant Infrastructure - Solutions CloudCommented:
Or if there is too many contacts, you can create a new connector from your organization to a partner organization. You include the different domains of the different companies you have in your distribution groups.

Let me know !
David XFSystems AdministratorAuthor Commented:
Turns out this was an issue with the configuration of the Barracuda spam filter. Thanks for the information on message tracking in 2016 - that led to the solution.
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