AOL fights spam, sure, but they have these great arbitrary rules in what you send their subscribers. Although I run a very clean double opt-in email list with conservative bounce policies (bounces quickly turn to suspended email attempts based on a variety of simple rules), I just had a bunch of "permanent" delivery errors with my Wi-Fi Networking News list. The crime? Including a link that used an IP address (a Google cached entry, as it turns out) instead of a full domain name for a Web site.
This is a bit heavy handed, although I'm sure it stops a lot of bad phishing attempts.
The extensive list of SMTP-embedded error codes that AOL has developed is impressive.