[lug] Making Postfix Play Nice (I'm not spam, really!)

[Ben Luey]

When sending out e-mails to our mailing list, I'm having a problem with 
some sites thinking I'm sending spam and blocking my mail server. I'm 
running postfix and the problem is that many of our subscribers have the 
same domain / mail server (say example.net). Postfix starts sending the 
users' mail. After ~10 emails in a row to example.net, example's smtp 
says "451 4.3.2 Please try again later (in reply to MAIL FROM command))" 
as a spam blocking protection. That's no problem and  Postfix respects 
that and tries again later -- for that one email.  But postfix may have 
50 more emails to send to that domain, so it tries to send them and gets 
the same 451. After 10 attempts in one second, example.net gets tired of 
giving 451 and blocks my mail server.

Is there a way to get postfix, when it gets a 451 message, to back off 
sending all emails to that domain? If postfix only tried 2 or 3 times 
after getting the 451 and then had the whole example.net queue wait 5 
minutes and try again, I think all would be happy. But instead postfix 
keeps pounding their mail server with ~50 requests in row that all get 
451. Then in 5 minutes, it tries all 50 requests again. Then in 5 
minutes it tries 50 requests again. Then it waits 10 minutes and tries 
again, but after the delay,cycle postfix tries EVERY email to that 
domain in just a few seconds. I've backed off the number concurrent 
connections max to 5 and that didn't make a different -- I think it 
isn't the concurrent but consecutive connections.

Ideas?

Ben