this is related to
JRA-931; because users can't be officially "disabled" email still gets sent to them even when they're no longer using the system.
for example, I recently left Notiva, and they're forwarding email to 'email@example.com' to my hotmail account. this is causing my hotmail account to fill up with JIRA notifications that I shouldn't receive (and no longer want)
either actually disabling a user (whether this means simply removing the user from all permissions and/or groups) or simply allowing the email field to be blank would be a great way to solve this problem.