Almost all SpamAssassin rejections fall into three straightforward categories:

1. The sender IP address has been blacklisted.

You could probably stop reading now. Only rarely do the other two cases come into play.

2. A message was pushed over the edge by a URL (or several URLs) that SA considers objectionable, for instance because they point to blacklisted IP addresses (see #1). This is uncommon, but does happen.

3. The message is very spammy. This is an extremely rare condition in the absence of #1 and #2. If you try and type a blatant spam on purpose, without cheating and looking up the rules and their scores, it will go through. I spent a lot of time on that exercise, and I do know how to curse in many languages ;-) It is amazingly difficult to get filtered on the basis of the text of your message (excluding URLs, hostnames and so on).

You would not be asking this question if #3 applied, so you can examine the URLs in the message to rule out #2, and conclude that the sending IP address got blacklisted, which is the main reason anything gets bounced nowadays. You cannot do anything about it other than wait and hope the blacklist is removed. The more public the IP address, the more likely it is to get and remain blacklisted.

  Eric