I think the problems are due to SPF, a rather complicated and controversial anti-spam mechanism. I just tried to adjust it, hopefully things work better. If SPF is too much hassle, I can also deactivate it completely.
Please drop a line in this thread if from now on you still do not get any notifications for subscribed topics.
An alternative to "topic subscriptions" is to use a news reader; each topic, and also the whole forum, have RSS feeds. If you already use other news feeds, this might be an elegant solution because it avoids all spam-filtering issues.