We have an Exchange 2013 on-premise server and seem to have an issue with emails sent to internal and external users at the same time.
The issue came to light because someone sent an email to 44 recipients, of which one was internal. None of the external recipients received the email. I checked the delivery report in the EAC and found the internal email marked as 'Delivered' and all of the external ones marked as 'Pending'. I checked the queues and there were none. I did some testing and sent an email to just one of the external addresses on the list, it arrived. I tried sending the email again to all of the recipients, the external ones all showed 'Pending'. I tried it again, but this time excluded the internal email address and all of the 43 external emails were immediately delivered.
So it seems that the issue only arises when we are sending to both internal and external addresses.
I then tried a test email to one internal address and one external address. The Delivery report says that the internal address was delivered immediately, while the external address is 'Pending' and gives more information saying: 'Message delivery is taking longer than expected. There may be system delays. For more information, contact your helpdesk.'. To add further mystery to this, the email was actually delivered.
So, I have two concerns:
First is seems that some emails sent both internally and externally are only arriving internally. This is a huge problem because I don't know how many have been affected. There may be many lost emails we don't know about.
Second, it looks like I can't trust the delivery report. It says pending for some emails which didn't arrive, but it also says pending for some which did arrive. That is no good at all.
For info the server is running Windows Server 2012. I have run a Microsoft Update to check if there are any to apply and the only Exchange one is a spam filter update, which I doubt has any bearing but I will apply when I get chance.