Quoting lengsc(a)coretech.co.nz:
the issue with yahoo mail servers is known and we ran into the same problem some time ago in germany. i just mailed my guys over there for confirmation but the way we dealt with it (at least i think we did) was to limit the connection rates to yahoo/xtra (config value in postfix, should be there in other MTAs as well). i'll post a more detailed thing when the germans woke up :-)
i just had a chat and the results are not so straight forward as i had them in mind. actually the rate limiting was no permanent soloution as our queues ran full over the time. we managed to whitelist our servers at yahoo finally which is obviously the best soloution. we have an overall mail traffic volume of about 2TB/month and holding the growing spool was a real pain. i had another look at the log lines posted originally and just wanted to mention that yahoo uses greylisting, however they also have that "Excessive unknown recipients" message, therefor you have to get whitelisted. greets lenz