Speaking of Yahoo, I and many others hate their rate limiting which is ridiculously low by default, something like 1 email per day. If you want more you need to fill in a form [1].
It took dropping an extract of our pflogsumm logs (showing very good delivery rates to numerous Y! competitors) on their C-level execs (all I could find) before I got to speak with a "concierge" who apparently expedited the process.
[1] http://help.yahoo.com/l/us/yahoo/mail/postmaster/bulkv2.html if I remember correctly