It's more than just that. My systems have recently been blacklisted by the blackhole list, and by barracuda, senderscore and spamhous because people who signed up to receive emails from my systems had dropped their email accounts without updating their website delivery settings, resulting in my server continuously hitting their now non-existant mailbox.
Their host systems reported to their anti-spam system who shared that info with all their other anti-spam customers, resulting in a sudden blockage of hundreds of domains.
Simultaneously, in an effort to thwart the sudden increase of spam due to the holidays, several anti-spam systems kicked their rules up a notch and now mandate that every mailserver IP have a valid RDNS record, and every mailserver have a PTR record and an abuse@ and postmaster@ account before they'll whitelist the domain.
In addition, Yahoo has been real bad about greylisting almost every domain until deemed safe. Read about it here:
http://www.ahfx.net/weblog.php?article=107
Comcast uses the SenderScore system which has a "best practices" page:
http://sendersupport.senderscore.net/bestPractices.php
It's getting tougher and tougher to run a bona fide mailserver these days.