Here are the steps to have Notes and Hexamail coexist:
Move Notes to another port.
1) You will need to be running Notes 4.61, and not r5. From the release notes for version 4.61, here is how you change the port:
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.