Here's a Wikipedia article for you: Greylisting.
After you've read that, then consider the error message I always get for my groups.google.com account when you disable mail sending to me due to "bouncing":
Bounce status Your email address is currently flagged as bouncing. For additional information or to correct this, view your email status here [link].
"Google tried to deliver your message, but it was rejected by the server for the recipient domain haxx.se by [mailserver]. The error that the other server returned was: 451 4.7.1 Greylisting in action, please come back later"
See, even the error message spells out what it is all about!
Thanks to this feature of Google groups, I cannot participate in any such lists/groups for as long as I keep my greylisting activated since it'll keep disabling mail delivery to me.
Enabling greylisting decreased my spam flood to roughly a third of the previous volume (and now I'm at 500-1000 spam emails/day) so I'm not ready to disable it yet. I just have to not use google groups.
Update: I threw in the towel and I now whitelist google.com servers to get around this problem...