I would like to disable Mail from loading.
It (and my Internet Provider AT&T) insist on keeping my old deleted sbcglobal email address. Of course, I have added my two preferred current email addresses.
Quit the program and stop using it. Find some other email application you would prefer to use. If you open Mail preferences and click on the General icon in the toolbar you can change the Default Mail Program to the email application you prefer to use.