We recently acquired another company with it's own domain and on prem exchange environment (just like the ours). We established a forest trust and that is working well however we are having some email issues. At this point I am not sure this issue is a result of joining the domains through the forest trust but it is a recent change.
Here's the scenario.
Mail from company.a.com can get to company.b.com without issue.
Mail from company.b.com to company.a.com does not go through.
In the queue viewer for company.b I can see the que for company.a.com with this error message
If company.b.com pings mail.company.a.com it returns the internal address of the exchange server.
If company.a.com pings mail.company.b.com it returns the company.b.com spam host (as it should).
Here is information from one of the emails that is stuck in the que on company.b exchange server.
Subject: test 2
Internet Message ID: <FBBAA7F55C821243B5B4F54AF
From Address: Administrator@company.b.co
Size (KB): 3
Message Source Name: FromLocal
Source IP: 255.255.255.255
Date Received: 10/7/2019 3:58:08 PM
Expiration Time: 10/9/2019 3:58:08 PM
Queue ID: company.b\32793
Exchange Routing Group (DWBGZMFD01QNBJR),CN=Routi
ng Groups,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Admin
istrative Groups,CN=company.b Organization,CN=Microsoft Exchange,CN=Services,CN=Co
I find the bit about routing groups interesting. Company.a doesn't have any routing groups but Company.b does... perhaps this is a legacy function from a migration.
Last bit of investigation I have found is this little bit.
company.a.com handles emails for other domains as well. company.b.com also sends email to those domains without any problem at all. It's only company.a.com that is having this problem. Again, I point to DNS on this because these other domains are not in dns at company.a.com. In other words mail.company.a.com exsists but none of these other domains do so they are still going to the spam host and not routing internally as mail.company.a.com does.