Exchange failing to route email after setting up forest trust

Lean IT
Lean IT used Ask the Experts™
on
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.

company.a.com
company.b.com

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 Screenshot_1.jpg.

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.
Identity: mail.company.b.com\32793\108813
Subject: test 2
Internet Message ID: <FBBAA7F55C821243B5B4F54AF90BE4319E0C68DC@mail.company.b.com>
From Address: Administrator@company.b.com
Status: Ready
Size (KB): 3
Message Source Name: FromLocal
Source IP: 255.255.255.255
SCL: -1
Date Received: 10/7/2019 3:58:08 PM
Expiration Time: 10/9/2019 3:58:08 PM
Last Error:
Queue ID: company.b\32793
Recipients:  administrator@company.a.com;2;2;;0;CN=SMTP Default,CN=Connections,CN=Exchange Routing Group (DWBGZMFD01QNBJR),CN=Routing Groups,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative Groups,CN=company.b Organization,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=company.b,DC=com

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.
Comment
Watch Question

Do more with

Expert Office
EXPERT OFFICE® is a registered trademark of EXPERTS EXCHANGE®
timgreen7077Exchange Engineer
Distinguished Expert 2018

Commented:
Add the email domain of company B to your accepted domains on company A and see if that helps. You can also create a send connector on company B to connect to the domain of company A and see if that helps.
IT Service Manager
Top Expert 2012
Commented:
On company A exchange server.. how you are receiving emails ?

1. Company B send connector properly configured to send the emails to company a server or IP address ?

2. Company A exchange servers receive connector or the email appliance properly configured to access emails from B server?

Telnet on port 25 from Company B to Company A sever and ensure no issues with the network access.

Author

Commented:
On company A exchange server.. how you are receiving emails ?
I don't quite understand this question.  Company A is getting all other email just fine... it is delivered after clearing our cloud based spam filter service.  Company B email is not making it into the cloud filter at all (they used to).

1. Company B send connector properly configured to send the emails to company a server or IP address ?
Nothing has been changed on the company b send connector.  It was working and now it is not.  What should be changed on the send connector in this configuration?

2. Company A exchange servers receive connector or the email appliance properly configured to access emails from B server?
I have not done this yet.  What would this configuration be?

Telnet on port 25 from Company B to Company A sever and ensure no issues with the network access.
That works fine.

Author

Commented:
I think I got it with a custom send connector!  I'll update with details once confirmed!!

Thanks

Author

Commented:
The solution was a custom send connector in company B.  I was clued into this from Rajkumar's post.  When he said how is mail delivered it got me thinking about the smart host (spam filter).  That's where I really needed to get mail moving to and a little bit of research led me to create a new custom send connector in company b for the address space of company.a.com.  I was then able to set up multiple destinations for company.a.com (my two MX records for company A).  Once that connector was set up I restarted the exchange transport service and mail started flowing.

Do more with

Expert Office
Submit tech questions to Ask the Experts™ at any time to receive solutions, advice, and new ideas from leading industry professionals.

Start 7-Day Free Trial