What the software does is validate registrations. It is ONLY functional during the registration process OR Kunena forum postings. Any other time, it doesn't do anything.
By default, what it does is check the IP address and email address of the registrant against various lists to see if they are reported as a bot or spammer. The internal list (which isn't exposed anymore) consists of over 300 disposable domains (e.g. trashmail.com). The other checks (unless you add domains/addresses to the white or blacklist) are done by querying other servers (e.g. Spamhaus, Botscout). For some of these servers you will need to sign up and obtain a key (these are Botscout, Project Honeypot, Akismet).
When external sites are checked, it is the specific email address that is checked. As you have seen, the vast majority of spammers are using email addresses from free email providers such as Hotmail or Gmail. This doesn't mean that all Hotmail or Gmail addresses are blocked, but only specific addresses that have been actively determined to be spammers.
Easiest way to test RV is to use a disposable domain to sign up with (e.g.
xx@trashmail.com will be blocked). This will test basic functionality and the internal list. Another check would be to use an email address blocked by another service (see
www.stopforumspam.com for a list of these you can use - any listed on their site should be blocked).
Hope the above answers your questions. If something isn't clear, just post back and I'll try to explain further!