Here are the steps to have IIS Mail and Hexamail coexist:
Move IIS Mail to another port
Two different methods for doing this are available:
1) Open the registry editor (regedt32.exe) to move IIS Mail port 26. The key you want to change is located at:
1) If using IIS v4, you can use the IIS management console.
Run the "IIS Management Console".
2) Click on the left-hand side, to select "Console Root / Internet Information Server / YOUR-SERVER-NAME / Default SMTP Site".
3) Right click on "Default SMTP site" and choose "properties". On the first tab, labeled "Default SMTP Site" you will find a section labeled "Incoming connections", and a field labeled: "TCP Port" which has a current setting of 25.
4) Change that setting to 26
5) Finally, go into the Control Panel, Services, then stop and restart the service named "Microsoft SMTP Service".
Install and Configure Hexamail
1) Install Hexamail
2) You will now need to set the SMTP Server setting in Hexamail. If you are installing Hexamail on Windows, the setup program will ask you for the TCP/IP and port of your mail server during the configuration Wizard, or you can change this by opening the administration interface and setting the port on the SMTP Server settings page.
3) Next, set Hexamail SMTP Relay settings to point to your newly configured port.