We have an environment that has 2 tree root domains - a.domain.com and b.domain.com. We migrated the users from b.domain.com to a.domain.com and added the b.domain.com UPN suffix in a.domain.com.  Our Exchange 2010 servers are in a.domain.com and server users with the email addresses joe@a.domain.com and joe@b.domain.com.

We would like to decommission b.domain.com and are wondering if the decommissioning of the domain will cause any email problems?  Is there anything that I need to look out for?

Nobody can promise that you won't have problems.  You could turn all the domain controllers off for a little while to see if anyone or anything is trying to connect to them.

Hi prindte,

If you have users still using the old @b.domain.com as EmailDomain.

You might want to add the relevant DNS entries in the DNS servers, if not present.

Configure an accepted domain within your Exchange organization as authoritative:

https://technet.microsoft.com/en-us/library/jj657734(v=exchg.150).aspx

Before you configure an accepted domain, you must verify that a public Domain Name System (DNS) MX resource record for that SMTP namespace exists and that the MX resource record references a server name and an IP address associated with your Exchange organization.

Also the autodiscover.b.domain.com need to be pointing to the correct server.

If you want to totally decommision the domain, then make sure you remove the domain from accepted domain in Exchange, and then remove them from all the users and later remove the DNS entries.

However doing so might give rise to NDRs when users in outlook tries to reply to old emails containing old domain addresses. Also sometimes due to cached contacts it would generate ndr. And there is no automated solution for users outside your organization replying to old emails.

Best would be keeping @b.domain.com as secondary and make @a.domain.com Primary for each user ac

Hi,

Make sure you have the related DNS records for a.domain.com and b.domain.com.

I suggest you can refer to Ed's suggestions to turn off all the domain controllers out of businesses hour and do some tests. 

If the mail flow work fine, we can decommission the domain b.

If you encounter some issues, you can post the error in the forum.

Regards,

David