Hi, I'm not sure if you're an experienced user or if you new at reading/posting to newsgroups? Do you consider yourself to be familiar enough with usenet to determine that the problem is with Astraweb and not something on your end (or at the ISP's end)?
Keep in mind that there is also a difference between subscription newsservers (which require logging in) and most ISP servers (which don't). A 500 error is a failure in communication. I've also found that ISP servers tend to be much faster at reading and posting messages, presumably because they don't have to go through the log-in sequence all the time.
Newsleecher is, of course, not a true newsreader, and until recent versions did not even support posting or reading news - only fetching headers and grabbing binaries. I suggest to try posting a text message with a traditional newsreader like Outlook Express or Thunderbird or any other *real* newsreader. Sometimes there can be a handshake problem between a particular client and a particular server, so it always helps to experiment with something new.
Sadly, Astraweb does not handle support questions on weekends --something the website fails to mention-- so if you already sent them an email, don't expect a reply until Monday afternoon. Since you use block accounts, you might also want to consider
www.Usenet-News.net since they offer cheap ($2) non-expiring block accounts with unlimited posting, and they also offer fast 24/7 tech support, so you can be up and running while still waiting for Astraweb to reply.