To change your email settings you need to start by changing them in the exMon database. You change them in exMon Administrator under General -> Configuration -> General.
To make sure exMon can send emails using your email settings you can test them by using the --testemail parameter
Now that you have ensured that your email settings work with exMon, you need to change configuration files on exMon execution servers. These settings are used in the event the execution server cannot contact the exMon database.
Best way to find all exMon execution servers is opening exMon Administrator and going through all the Execution Connections.
Remote to each Execution Connection and open the file epcmd.exe.config and change the email settings.
When this has been done, exMon should no longer use your old email settings.