Small businesses usually have one Exchange connector, the one that sends SMTP mail to the Internet. Some may have fax connectors, but that's it. During migration or in co-existence scenarios a sophisticated connector such as the Lotus Notes one might be deployed. However, in my years of consulting on Exchange projects I've encountered some scenarios where the plain vanilla SMTP connector is required.
What do SMTP connectors do?
Exchange 2000 and 2003 servers have a strong revised SMTP engine that is able to receive mail without any configuration. The SMTP connectors basically simply tell that SMTP engine how to route mail.
The basic connector is the one used to send mail to the Internet. The Address Space tab is typically configured like this:
This tells Exchange to send all mail not intended internally (as indicated by the recipient policies) through the connector. This is indicated by the asterix sign ("*").
Setting up a second SMTP connector
Let's say our SMTP domain is domain.com. WeYou setup a sub-domain for the people using the UNIX mail server with the name unix.domain.com. Now we would like to configure our mail server to relay to the UNIX mail server, so you set up a second SMTP connector.
The connector is configured to relay all mail through what is commonly know as a "smart host". This means that instead of using DNS to find out MX records for the mail all the mail is sent to one SMTP server that decides for itself how the mail is routed. The IP address of the UNIX mail server is entered here in brackets.
On the address space we specify the domain we wish to relay for using this connector.
After pressing "OK" we now have two connectors. Should this work? Actually, no. Since both connectors have the same cost, the results can be unpredictable because the address space "*" of the out connector also includes the address space "unix.domain.com" because it includes all the domains. This means that mail intended for the unix.domain.com domain might still be routed through the out connector to the Internet.
To make sure that mail is routed through the right connector we change the cost on the Internet SMTP connector to something higher, such as 25.
Exchange will first check the connector with the least cost for routing information, in this case the connector to the UNIX mail server and route the mail correctly.
Knowing how to deal with routing can be handy when you have multiple mail systems. It can also be useful when you're migrating to a new mail system and want to switch between an old connector and a new one. Just remember how to play with costs and do some mail tracking to make sure that mail actually goes through the right connector.