miniBB provides possibility of "subscriptions" to the topic, however, user WILL NOT receive each and every reply until he views the topic on forums actually.
It means, for example, you subscribe to this thread, then I answer you, and you get notified about my reply on the email (you read not the full message by the way, but only the first 150 symbols). Then if somebody else answers, and you still haven't clicked on the topic link and haven't read the message on forums, you WILL NOT get this notification. But if you have read the topic already, you WILL get it again. | 
