Recently we've had a number of spam emails sent to the mailing list, which Google hasn't caught. We apologise for this, but we ask you to just ignore them. As soon as we spot the ones that make it through Google's net, we delete and ban the user.
Unfortunately, the only other option would be to send all new users' first posts into a moderation queue before allowing them to post. With a list as busy and responsive as this one, we think it would be detrimental to make new users wait until somebody looks at the moderation queue.
Thanks for your understanding. Fortunately, the levels of spam are pretty low and don't interfere too much with the group. If things get worse we will revisit this policy.