So what happens if the user gets the email, but doesn't click on the included 'link' for some days? Does the subscription terminate?
It is definite that the user must check the topic to continue the subscription. I'm out of the working desk now and couldn't remember it exactly, but the topic either should be visited by clicking the email link, or just any other way on the forum (will say it more precisely tomorrow). Btw this option is so rarely requested that I must actually re-check if it truly works by now...
If more messages are added during the period when the topic is not visited, they will not arrive
as soon as the topic is checked.
The user should not know about this background. If it receives a subscription email, it's already a reminder to check it out. If this reminder is ignored, the subscription terminates. That's all to know.
This all is done to save resources on your server on wasteful users. Many of them subscribe and then forget the topic. In this case, they should not be notified anymore — each email takes *a lot* of resource on your server! Overall, it also could produce the spam effect, so they are set as bulk at some stage on the client server.