Now you have to have the right reverse-DNS
Update: Several of the spam bounces of this sort that I got were traced to the same anti-spam system, and the operator says it was not intentional, and has been corrected. So it may not be quite as bad as it seemed quite yet.
I have a social list of people I invite to parties. Every time I mail to it, I feel the impact of spam and anti-spam. Always several people have given up on a mailbox. And I run into new spam filters blocking the mail.
Perhaps I'm an old timer, but I run my own mail server. It's in my house. I read my mail on that actual machine, and because of that, mail is wicked-fast for me, as fast as instant messaging for many people. (In fact, I never adopted IM the way some people did because E-mail is as fast.)
They're working to make this harder to do. Many ISPs won't even let you send mail directly, or demand you make a special request to have the mail port open to you. I'm bothered by the first case, less so by the second, because indeed, zombie PCs send much of the spam we're now getting.
Because I send mail from the system, I also web surf from it. And while it's not a serious privacy protection, I decided I would not have a reverse-DNS record for my system. That way people would not see "templetons.com" in their web logs whenever I surfed. It's not that you can't use other techniques to find out that the address is mine, but that requires deliberate thought. Reverse DNS is automatic for many web logs.
Soon more and more sites would not take mail from a system without reverse DNS. Because I get my IP block from a small ISP, he does my reverse DNS, and I asked him to make one. He made one like many ISPs do, built from the IP numbers themselves. As in ip-nn-nn-nn.ispname.com.
But soon I saw bounces that said, "This reverse DNS looks like a dialup user, I won't take your mail." So I had him change it to a different string that doesn't trumpet my name but doesn't look like a standard anonymous reverse DNS.
But now I'm getting bounces just because the reverse DNS doesn't match the name my mail server uses. There is no security in this, any spammer can program their mail server to use the reverse DNS name of the system they have taken over. But I guess some don't, so another wall is thrown up, and those people won't get invites to my parties.
This one is really stupid because it's quite common for a single machine to have many names and serve many domains. To correct an earlier note, it is possible for an IP to have more than one PTR reverse DNS record, though I don't know how many applications deal with that. And that screws these mailers. There is no need to look at reverse DNS at all.