In Exchange 2007, we know that email routing is directly tied to Active Directory Sites and Services. A mailbox server in AD site A, will only be able to route using a Hub Transport server in the same AD site. It can never fail over to a Hub Transport server out of it’s site, so we need to take care and build resiliency for our Hub Transport servers in each site.
Exchange 2007 is also smart, it will round robin load balance, automatically, between the 2 Hub Transport servers. This means, that if one of the servers goes down, mail flow doesn’t stop, it simply fails over to the remaining Hub Transport server without issue. There is no need to do anything in this case, all of this is done automatically behind the scenes…..but what if you don’t want it?
What if you need to isolate a HT server, for troubleshooting purposes? Lets say you have three Exchange 2007 servers in your site, NYMB01, NYHT01 and NYHT02. NYHT01 and NYHT02 are Hub Transport/Client Access Servers. NYHT01 has been acting up, and you want to run some isolated troubleshooting steps on it, but that means it will still be up, but you don’t want it to be serving email. Problem right? The MB server automatically will discover the HT server and use it for routing of email. There is a way to stop this.
In ADSIEDIT, navigate to Configuration->Services->Organization Name-> Administrative Groups->Exchange Administrative Group->Servers.
Now, right click on any Mailbox Role Server, and select properties. Now search for a value named
This is the value that lists which servers the MB server can use to route email. By default, it will be blank:
You can add distinguished names of the Hub Transport servers you want the MB server to only use, and this will stop it from discovering HT servers dynamically. You can edit this value, or use a powershell command:
Set-MailboxServer –identity “NYMB01” –SubmissionServerOverrideList “NYHT01”
This will force the MB server, NYMB01, to only use NYHT01 as its HT server. To add multiple HT servers, just separate the values with a comma.
Now look at the same value in AD:
Now, NYMB01 will only use NYHT01 to route email. Remember though, if NYHT01 happens to fail, your MB server will not be able to route email to any HT server that is NOT ON THE OVERRIDE list, so if you are going to do this, make sure it’s temporary, and you have redundancy.
To change it back, simply clear this value of all specified servers!