Guess which is one of the IP addresses used by Yahoo Groups?
A quick look reveals that they're not alone in this, but most such lists don't think it should be included.
Unfortunately(?) SORBS is used by several UK ISPs as part of their anti-spam effort, so they're happily rejecting - with a 'hard bounce' which Yahoo interprets as "Don't send me any more email, ever" - Yahoo Groups emails.
The solution is to log into Yahoo and give it another email address to send stuff to. Gmail/Googlemail appears to be OK. Then look up the pages for the various groups you thought you were on, and see what you've missed, grrrr.