This is a bit of a specific topic, as it only affects people who are trying to use the MTA in their cluster (for mail delivery back to 5.5 in mixed-mode, or to foreign systems not connected with SMTP, etc) *AND* have >4 nodes in their cluster.
There’s a new KB out that will help you with some performance tuning in this scenario: http://support.microsoft.com/kb/899302. This is a KB that is near and dear to my heart, as I was the original case owner on a group of these issues before I moved over to the product group.
In any case, what happens is that if you have more than 50 MDB databases sharing a single MTA, some scalability settings in the MTA are not set correctly by default and need to be increased. How do you get 50 MDB databases sharing a single MTA? Well, in an Exchange 2000/2003 cluster, there is but a single MTA for the entire cluster. And in Exchange 2003, you can create up to 8 nodes (up to 7 Exchange Virtual Servers) in that same, single-MTA cluster. Each EVS can then host up to 20 MDBs. So, in theory, you could load up an Exchange 2003 cluster with 140 MDBs.
Don’t do this. It’s a bit like RAID5 disks. If you have 8 disks and you set aside a single disk as parity for RAID5, and then you proceed to fill up to 100% capacity the usable disks… you’re sunk if you suffer any physical disk loss in a 7+1 configuration. You don’t want to spread yourself that thin!
Anyways, back to the topic at hand: you may find yourself getting event 3121 logged in your application log. This is telling you that you’ve hit the maximum # of associations for the MTA (ie – too many stores are trying to talk to the MTA at once). The KB article walks you through the one — and only one — registry change to make in this case. Don’t make other changes to the MTA settings in the registry, or set the reg key to some higher value than recommended by the KB.
With that registry key in place, you’ll be able to exceed the 50 MDB limit and increase the scalability for these very-large Exchange 2003 clusters.