I've seen Exchange bounce an email after receiving a 450 on the first attempt, although it's quite possible that was due to clueless Exchange admins. Postfix d

I've seen Exchange bounce an email after receiving a 450 on the first attempt, although it's quite possible that was due to clueless Exchange admins. Postfix default is 2d, Exim 3d, and sendmail 5d, last I knew.