We have a script running that deletes subscriptions that fall into this category :-
a) Email notification (for thread subscription, PM notification) is turned on
b) If email bounces back for a set amount of times, members subscriptions are deleted and the PM notification status is reset to pop-up.
This is because our webmaster account is regularly filled with bounced emails for Thread Subscription notification or PM notification via email when the email address is no longer valid.
If you want to preserve your subscriptions, make sure that your email address is up-to-date and valid.