Greg Lindahl wrote:
7E7E824F81 634 Wed Mar 1 09:03:21 bar@baz.com (host mx3.mail.yahoo.com[67.28.113.10] said: 451 VS5-MF Excessive unknown recipients - possible Open Relay http://help.yahoo.com/help/us/mail/spam/spam-18.html (#4.4.5) 205.217.153.43 (in reply to MAIL FROM command)) foo@yahoo.com
I find a lot of stuff by Googling "VS5-MF Excessive unknown recipients", apparently Yahoo may be doing something that is not RFC-compliant, so your MTA doesn't know that it should stop trying to resend the message:
http://forum.futuresoft.com/forum/Default.aspx?g=posts&t=248
(FYI, I'm not an open relay, and I'm not in the RBL's you suggested I check.)
I'm not an open relay either, but it would not be the first time that a server that is not an open relay got on an RBL either--I have seen that happen at least twice. Also, there are some st00pid RBLs out there run by people who don't know what they are doing.
Also, it doesn't sound like the problem in your case, but it's possible for a server to have eg. a vulnerable formail script or the like, so it is not an open relay per se, but still routing spam.
Jonathan