Here are the steps to have Exim and Hexamail coexist:
Move Exim to another port
1) Locate the daemon_smtp_service option in your Exim
2) By default, this option is unset. You should set it to another port number besides 25. In this example we will use port 2500.
Here is the Exim documentation on this feature:
This option specifies the numerical port number or the service name equivalent on which the daemon is to listen for incoming SMTP calls. It is overridden by `-oX' on the command line. If this option is not set, the service name `smtp' is used.
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.