I've known Y! error 4.16.50 to be because of forward and reverse DNS entries for the Mail Server/Firewall not matching. After correcting the reverse DNS, it's been fine since. I also know that the commentary: "generating substantial complaints from Yahoo! Mail users" is just plain false (at least in this particular case). This being because, there was only one xxx(a)xtra.co.nz xxx(a)yahoo.xxx address in the logs (going back over several months before the problem arose), and this guy actually wanted to receive the data being sent to him... I suspect the 'mismatching dns error' was just cloned from a previous error code and not fully updated with the correct text. Paul. 2010/1/6 Mark Foster blakjak(a)blakjak.net
If you actually search for the specific error (4.16.50) on the URL given i the bounce, the result is this:
http://help.yahoo.com/l/us/yahoo/mail/postmaster/errors/postmaster-21.html
Note it specifically uses the phrase "emails from your mail server have been generating substantial complaints from Yahoo! Mail users" - ala people are hitting the 'report as spam' buttons and some internal Yahoo magic is being applied to the IP as a result.