Musings from a small IPP

Things have changed a lot in the 30 years I’ve been doing e-mail systems.  In those first heady days, establishing a transport fabric was hard work, and getting a message through over a mixture of uucp, decnet, snads, and arpa protocols a miracle of managing complexity, and co-operaion.  Now in February 2010 I reject 92% of the messages offered for my customers.  Strangely few of these rejections result in contacts from customers or their contacts  about false rejections — far more contacts relate to the SPAM we’re still letting through.  This week though I’ve had 2 contacts about mail getting rejected.  Both have the same underlying cause — the sending machine is not credible as a legitimate mailhost.

So what makes a credible mailhost?

1. Mailhosts have fixed IP addresses
2. Reverse DNS (when you look up the name associated with the IP address) gives a ‘real’ hostname, not a DHCP pool name.  For instance if I look up a host which has an IP address of 10.128.192.3 I would be looking for a name more like ‘mail.example.com‘ than ‘3-192.manchester-dsl.co.uk‘
3. The DNS lookup loop is complete.  That means that the name I get from looking up your IP address should look up in turn, and get me back to the IP address I started with.

None of this is even remotely rocket science.  As a mathematician might say, they are necessary, but not sufficient  to establish credibility.  Failing this test makes your server look like a virus-infected spam-bot.

If you’re trapped with a service provider who will not give you a fixed address or good reverse DNS then you have to either use the provider’s mail service for your outgoing messages, or a 3rd party who can offer a mail-submit service (such as we do).  Ultimately of course you should change provider to one which will provide the infrastructure you need to do the job you want to do.