"Any responsible SMTP relay server will not relay spam." - I respectfully take issue with this rather basic statement.

I consider our network to be 'responsible' but when our hosting clients blindly forward all email from an account with us to their AOL account (for instance), it causes problems that are broken down here-

1) client sets up forwarding for an account we give him to his AOL account

2) client receives spam; it is automatically forwarded to his AOL account.

3) client sees the spam at his AOL account and marks it as such, and AOL ends up penalizing US, because we are the last mail server to send this message, even though we aren't the spammer.

So it can happen to 'responsible' providers, too.

The main idea behind reverse-dns for mailservers is that a 'reputable' server would have an admin that would spend the time setting up a reverse-dns entry for the IP, but a hacked box or temporary spam site wouldn't. While this simple logic on the surface makes sense, this is obviously not a working model.

And while smaller ISP's are starting to adopt this policy, you can thank the largest providers for instituting this. The lamest thing is that spam has gotten so much worse over the years, yet they still cling to this practice as being a preventive measure-- but given the increase in spam I'm not sure they could prove it's really been that effective.

A question- would it really hurt to have the reverse-dns entry elucidate that the host is, in fact, a mail server? I know the basic idea is that you don't want to volunteer information, but from what I can tell, it's not only not the only way to identify a host as a mail server, but it's also not the preferred way... are we basically holding onto obscurity for nothing? When can we tell the emperor he's naked?