Recently migrated from Exchange 2007 to 2013. A large distribution group (about 1200 entries) is sent email once or twice a week. All members of this distribution group are contacts with external email addresses. After the migration emails to this group no longer go through. Looking in the message tracking logs, I can see messages to this distribution group are flagged as "TRANSFERRED". Internal mailboxes that are also on these emails receive them without issue. Apart from that no information in available.
No NDRs are received.
I turned on verbose logging on the send connector via the EMC in hopes to get a better handle on where these emails actually go, but I did not find anything. It is not clear to me where this might be logged in any case, so I may not have found the correct log or need to turn on more logging (maybe via PowerShell?)
Also set some message restrictions settings to allow up to 1500 recipients in case that was the issue.
As a test today I sent emails to contacts directly (without specifying the distribution group) and they went through fine up to about 700 at once. I should have another email to send to the entire group later this week. I have contacted the our 3rd party spam vendor who says that this message is never received by them. It appears that Exchange gets the email directed to the group and then just ... drops it?
There must be a log somewhere to tell me why distribution group members never get the email because it appears it never leaves the Exchange Server.