Question : Reverse Lookups - Spam - DUN and DUL

i support several small networks in the UK all have a few things in common.

ADSL + static IP
Small Network + Exchange Server

For ease of use I have the exchange servers using a pop3 connector rather than SMTP mail configuration. This means that port 25 does not have to be open for incoming traffic, which is a major security benefit.

To send mail I have the servers configured to send using smtp and dns.

Each site has it's own domain name that it sends mail from. As these are very small companies they have their website hosted by a 3rd party.

Their internet access is usuall ADSL, provided by BT or some such ISP.

The problem we have is that more and more frequently sent mail from these sites is returned by the receiving mail server with a variety of messages. Often it's this kind of thing

There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            dr.btopenworld.com #5.5.0 smtp;550 Denied by policy: Sender is listed on DNS-based RBL.>

Of course this means that the mail server is using some sort of referall list to check for blacklisted IP addresses.

Further investigation reveals these blacklists just randomnly perform reverse dns checks against the incoming email domain name and the originating IP address. All very noble but as you can see blocking for this reason is incorrect in this case. The work involved in removing IP addresses from various blacklists isnt worth it so..to the question.

Is it possible to fix the reverse dns so it points to the correct email domain, whilst the incoming emails still get sent to the pop3 host. While I have a good understanding of IP, I'm not 100% au fait with how mx records and reverse dns works.

Please dont mention smarthosts - I have tried that as a fix and all i get is disappearing emails with no traceability.

Thanks in advance

Answer : Reverse Lookups - Spam - DUN and DUL

Hi sloopeth,
Assuming the mail server of a company is called mail.company.com then you need to check the following:-
1) The mail server is announcing itself as mail.company.com
2) A DNS lookup on mail.company.com results in the servers IP address.
3) A reverse DNS lookup on the IP results in mail.company.com. In order to make this change you will need to contact the ISP directly as only they can do it.

Next go to http://www.rbls.org and enter your IP address and check to make sure it is not on any other blacklists.