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ravenstar68's avatar
ravenstar68
Very Insightful Person
9 years ago

Ravenstar's Email FAQ's

The aim of this thread is to collate some of the stickies I have made in the past into one thread. In here you will find the following FAQ's Understanding Email Block Lists SPF, DKIM, and DMA...
ravenstar68's avatar
ravenstar68
Very Insightful Person
9 years ago

Generic/Dynamic host names prohibited

I'd like to assist businesses who are trying to send emails and are getting rejected with the above message by explaining it in more detail and what you can do to resolve this particular error.

Many ISP's might give a Generic host name to a customer's IP address.  This means that when you do a reverse lookup using dig or nslookup, you might get something such as:

Default Server: cache1.service.virginmedia.net
Address: 194.168.4.100

> set type=ptr
> 94.173.xxx.xxx
Server: cache1.service.virginmedia.net
Address: 194.168.4.100

Non-authoritative answer:
xxx.xxx.173.94.in-addr.arpa name = cpc35-sutt4-2-0-custx.xx-x.cable.virginm.net
>

(Note this is my home IP so I have masked some of it out somewhat.)

While the above is for a residential connection I have seen similar for static business connections on both bt and comcast.

If you are running a mail server however the PTR should not point back to a record of this form.  If it does Virgin reject the mail.  

While some other servers may accept that mail it should be noted that mail is more likely to end up in a customer's spam folder.

It is important when running a mail server that the ptr record for the IP address points back to the fully qualified domain name (FQDN) of the mail server.  The server should also quote it's FQDN in it's HELO/EHLO exchange.

For example if we take a real world example from a mail I recently received.

Received: from mail-qt0-f199.google.com ([209.85.216.199])

The mail server has greeted Virgin's server with it's FQDN and Virgin's server has also noted the IP address of the server.

Plugging the IP address in to nslookup.

> 209.85.216.199
Server:  cache1.service.virginmedia.net
Address:  194.168.4.100

Non-authoritative answer:
199.216.85.209.in-addr.arpa     name = mail-qt0-f199.google.com
>

When you are assigned a static IP address then you should by rights have been given control of the reverse DNS zone for the IP addresses - however if not you may need to ask your ISP to set the reverse DNS to match your servers FQDN and wait for it to propagate across the web.

Ravenstar68