subscribe and unsubscribe from email via zen_mail code

We are running a third party email program. To be able to automatically update the email list for that program I need to send emails from the user that is subscribing or unsubscribing to a specific email address with a subject of subscribe or unsubscribe. I have modified various files in my custom folders and the subscribe option is now working both from a side bar entry form and from the create account page. The problem is that I haven't been able to figure out where to place the zen_mail statement for the unsubscribe. I've been going at this trial and error for some time now and can't seem to find the correct files. Is there a schematic that might show me a flow of a newsletter subscribe/unsubscribe? There is only one check box available on the "my account" page and it seems to work like a toggle off/on but I don't know how to tell it when to send a subscribe or send an unsubscribe email.
Whenever the newsletter field is updated by zen cart, I need the appropriate email to be sent via zen_mail. Can anyone tell me which files I should be customizing?

If you're talking about activity done on the index.php?main_page=account_newsletters screen, then the file is /includes/modules/pages/account_newsletter/header_php.php