Reverse DNS and emails....

[techmonkey]

I have a physical machine called server.mybiz.com - and that machine has 50 virtual accounts on it - all having unique domains. My understanding is that there is just one mail server for the machine that will have the address server.mybiz.com.

So we have this local ISP who's spam fighting strategy is to lookup the reverse DNS for every email - and to reject any email whos mail server address does not match the senders address.

Let me see if I understand this. This strategy will only allow emails through that are from ISP's or from places like Yahoo, Hotmail etc. Anyone who has a web site on a shared server and uses mail from that account is going to get rejected. Is this correct?

[Chazzy]

But whats the difference between you having a mailserver on a shared server and hotmail's mailserver(bearing in mind they will have multiple with different IP's).

Ive never heard of an ISP doing what your saying and would think it would be quite detrimental ...

[techmonkey]

But whats the difference between you having a mailserver on a shared server and hotmail's mailserver(bearing in mind they will have multiple with different IP's).

Ive never heard of an ISP doing what your saying and would think it would be quite detrimental ...

But I would guess that all those hotmail IP's would resolve to some variation of the hotmail.com domain name. But I don't know a lot about the details of DNS so I really don't know.

[Chazzy]

Im not a DNS expert either but surely for them to be rdns'ing and only allowing things like hotmail etc then there would be a massive list of do's and dont's ... it would be difficult to manage and a lot of companies would get pissed off. ((its majorly against the ideas of email)).

Surely though, domain1 would have a mail address of mail.domain1.tld and domain2 would have a mail address of mail.domain2.tld which resolve to the IP of server.mybiz.tld as opposed to server.mybiz.tld .... I mean your customers wouldnt have their mailserver set as server.mybiz.tld ... it would be mail.mydomain.tld which even though it resolves to server.mybiz.tld's IP.

Hope that makes some form of sense

[techmonkey]

Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses. One of the applications of reverse DNS is as a spam filter. Here's how it works: Typically, a spammer uses an invalid IP address, one that doesn't match the domain name. A reverse DNS lookup program inputs IP addresses of incoming messages to a DNS database. If no valid name is found to match the IP address, the server blocks that message.

Although reverse DNS is fairly effective for filtering spam, it also sometimes blocks valid e-mail, at least in the current technological environment. A number of problems, including network delays and improperly configured networks or servers, can prevent legitimate messages from getting through the filter. In January 2003, AT&T WorldNet started using reverse DNS in conjunction with other anti-spam software. The company was forced to remove the filter just 24 hours after it was deployed, after subscribers reported that messages were going undelivered.

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If your server is my.server.com then every virtual host on the server is going to send mail from my.server.com - not from client.domain. Another problem is with people having email addresses on a server like this one - but their ISP does not allow relaying so they have to use the ISPs SMTP servers. These emails are going to show up being from joe@client.com - but the reverse dns lookup will show the sender as mail.bigISP.com.

The article above states that the reverse DNS lookup method will fail due to "improperly configured servers" - but it seems that there really is no way to have the sending IP match the senders domain in these instances.

Edit: this vBulletin WYSIWIG is pretty buggy eh? Can't seem to just have part of the post italic.....

[Chazzy]

Ahhh ... of course because xxx.xxx.xxx.xxx will rdns as mail.mybiz.com .. Thats kind of stupid as AT&T found out.