Regarding the default/internal MailListener, the documentation states:

This listener is usually always turned on in JIRA - see Email Notifications for more details. If you want to write more complex or more specific notifications, you can disable the internal MailListener and add your own.

However, as the MailListener is marked as 'internal', it is not possible to be remove it.

At present, it is necessary to recompile the MailListener class and set isInternal to false.

Need to allow users to delete/remove the MailListener through UI.