Here are the steps to have SLMail and Hexamail coexist:
Move SLMail to another port
1) Open up the SLMail control panel
2) Go to the tab labeled "Access".
3) Locate the entry labeled "SMTP Port".
4) This is currently set to 25. Change it to be 2500.
5) Restart SLMail.
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.