Hi Michael!

The RSS is no real solution, because it requires the forum to be open for all.

That doesn’t matter. My forum is also limited to registered users (precisely: to post, but not to read) and the RSS is still available to everybody.

Is is really so difficult to give all users the right to recieve emails on new entries?

No, it isn’t. I implemented this option in my forum where the majority of members come from developing countries. ~25% are still using IE6. RSS-Reader – never heard of. Now for the big but: I made this statement in the instructions:
“Please tick the E-mail notification box only if you cannot use the RSS Feed. Using the notification may result in a lot of e-mail traffic, not only to you personally, but the forum’s e-mail server may be classified as a bulk-mailer / spammer and blacklisted – which could affect all users.”
Luckily only less than 1% of users have opted in (which still results in up to 300 e-mails / day).