In addition to having all the above properly setup so that your mail
servers appear squeekly clean from the outside, make sure they ARE
squeeky clean - on the inside. You may wish to raise this issue on the
spam-l mailing list:
* Make sure your mail servers are squeeky clean. Forward and
reverse match, valid MX's, they report their own name in SMTP
headers, no "untrusted sender used -f", etc. Valid abuse@
for the machine name, and the parent domain are essential.
Valid contacts for the domain and IP block are helpful.