We're having an issue where users that are members of various distribution groups are not reliably receiving messages. We have a newsletter that is sent out weekly by one user. They use 4 internal distribution groups (some of them nested) plus external email addresses that add up to around 1170 recipients. We have around 280 internal users. Of those 280, around 100 are delivered, and 170 are not delivered but show as "pending" in the delivery report. External users seem to be receiving messages fine. 

I tried doing a trace on myself, as I didn't receive the newsletter that was sent out yesterday.

$Temp = Search-MessageTrackingReport -Identity lorraine.user -Recipients jsmith@domain.ca

[PS] C:\Windows\system32>$Temp | %{Get-MessageTrackingReport -Identity $_.MessageTrackingReportID -BypassDelegateCheckin
g -ReportTemplate Summary }

The missing message was not in the list. It should have been the first result.

Here is an example of how one of the pending delivery reports look like:

Pending
3/4/2015 4:12 PM s8ex1.domain.ca
The message has been transferred from s8ex1.network.domain.ca to S8EX2.network.domain.ca

Group Expanded
3/4/2015 4:12 PM s8ex1.network.caedm.ca
The list of members of the group "Camps - DG" was expanded so that the message can be delivered to each recipient

Pending
3/4/2015 4:13 PM S8ex1.network.caedm.ca
The message has been transferred from s8ex1.network.domain.ca to s8EX2.network.domain.ca

3/12/2015 12:55 PM s8ex1.network.caedm.ca
No further information is available about this message because the logs are no longer available.

Environment:

Exchange 2013 CU7

2 Servers running Windows Server 2012 in a DAG

Hi,

According to your description, I understand that message cannot be delivery to all internal recipient when send one newsletter to 4 internal DG and external account, however external users receives fine.
If I misunderstand your concern, please do not hesitate to let me know.

Pending means either the destination for the e-mail message hasn't yet been found or the message could be pending approval.
I want to double confirm whether the 170 users could receive the message or only cant receive this message.
Is there error message get back to sender after some time? Please try to send message to all 4 DG and try to reproduced this issue.
Then help to collect relevant event log and protocol log for further analysis.

Thanks

Since I last posted we have sent out several other emails to our internal distribution groups. It seems random but some users receive the emails, and some do not. There are no security settings that prevent the sending user from sending to any of the distribution groups. Issues seem to arise when sending to multiple distribution groups. When sending to a single distribution group, messages seem to always go through. 

What logging can I enable that will figure out why they are stuck in pending?

Hi,

Sorry for delay.
Please try to use Protocol Log as I mention above to get more distinct error message for further troubleshooting.

Besides, I find an article about Troubleshooting Mail Flow and SMTP, for your reference:
https://technet.microsoft.com/en-us/library/aa996805(v=exchg.65).aspx

Thanks