Co Exist With Another SMTP Server
In some cases it would be useful if your Exchange server could co-exist with another SMTP server. This could include another Exchange server, a UNIX machine or a List Server.
This is simple to do if you have multiple IP addresses, but what if you only have one and are also on a dynamic IP? How can you get it to co-exist with Exchange when you can only direct inbound email to one point.
Use Exchange to manage all email, redirecting it to the other SMTP server as required. By using this technique you can have multiple SMTP servers on the same IP address.
External Network (Internet) Configuration
If you are using different domains then point their MX records at your existing Exchange server's external IP address.
You can also use your own domain - perhaps with a sub domain. If this is the case create MX records for your sub domain also pointing at the Exchange server.
Internal Network Configuration
After making the changes on your external configuration you should replicate them internally, except use your internal IP addresses and point them at the other servers.
Your Exchange server is known as mail.example.com
You also have a list server known as lists.example.com
And a unix machine known as mail2.example.com
These entries need to be made in your internal DNS so that Exchange can route email to them correctly.
Create a new SMTP Connector in Exchange.
On the General tab, change the option to "Forward all email through this connector to the following smart hosts" and enter the IP Address or DNS name of the other server in the box for Smart Host. If entering an IP address it needs to be enclosed in [ ]: [192.168.1.1]
On the "Address Space" tab, remove the default * and enter the domain name that you wish to use (lists.example.com).
Enable the option "Allow messages to be relayed to these domains"
All other options can be left as default.
Q: I have a dynamic IP address for my internet connection. How can I use Exchange with this configuration?
A: We have a dedicated page for this here.