[_urga]

Did you check the reputation of your /24 neighborhood?

I have seen Microsoft blocking entire /24 neighborhoods on the basis of a single IP sending spam.

You were probably hit by that as collateral damage.

[ryan-c]

Yes, I did. It was clean as far as I could tell. It was not just Microsoft, I also had issues with Yahoo, AOL and some others. The block I'm in is listed in some RBL called "spamgrouper"[1] but as far as I can tell nobody pays attention to it. No other lists showed any trouble.

1. http://www.spamgrouper.to/