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@xtra.co.nz xxx@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@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.