If you run an outbound SMTP email server, then there’s a number of things you should be doing to ensure smooth sending of your email. All of the things below are fairly straightforward recommendations that are either specified by RFCs or as general best practice.
- Ensure your forward and reverse DNS match –
Also called Forward Confirmed Reverse DNS, having valid and matching forward and reverse DNS is one of the first recommendations in RFC 1912 (“Make sure your PTR and A records match”). It’s a sign that the system administrator understands at least the basic RFCs. It also helps to avoid spoofing of your systems by spammers.
You need to ensure that the IP address you are testing is the “edge” one that your email server connects to other servers with. In most cases this is obvious, but you might have a machine with multiple IP addresses, or you might be behind some sort of NAT system, in which case the apparent IP address will be the NAT router IP address, so make sure you are testing the right IP address.
There’s a tool to test that your forward and reverse DNS match here. Or you can do it easily via Linux command line tools. For instance, here’s the forward & reverse DNS for one of our outgoing hosts.
$ dig +short out1.smtp.messagingengine.com
$ dig +short -x 184.108.40.206
Note how out1.smtp.messagingengine.com -> 220.127.116.11 and 18.104.22.168 -> out1.smtp.messagingengine.com, this shows that forward and reverse DNS match.
If you’re using an ADSL connection or similar, then make sure you get a static IP (most ADSL providers have this option, it may cost a little bit more) and make sure you can get the reverse DNS changed (also known as setting a PTR record – your ADSL provider will have to do this, and not all offer it, so check with your provider first before signing up)
- Ensure your HELO string matches your reverse DNS
When your SMTP server sends email, it has to announce it’s name in the HELO or EHLO command. Since you have your DNS setup correctly, you have a fully-qualified domain name (the reverse DNS name), so you can follow RFC 2821 and use it as your HELO/EHLO string:
The argument field contains the fully-qualified domain name of the SMTP client if one is available
Doing this provides another level of verification that your server is who it says it is.
- Don’t use Sender Address Verification
At first glance, Sender Address Verification (SAV) seems like a good idea. Because SMTP doesn’t include include any intrinsic way to authenticate the MAIL FROM address, you just connect to the appropriate return host and check if the site will accept email for that address.
Unfortunately SAV creates more problems than it solves. As noted by others, it’s easy to work around SAV, spammers just send with a valid MAIL FROM address. Given they’re already spamming lots of valid addresses, they have lots to choose from.
For spammers that don’t use valid MAIL FROM addresses, the result will be that your system ends up looking like it’s attempting to attack other systems. For instance, say a spammer sends you 1000 emails with forged and invalid @fastmail.fm MAIL FROM addresses. To check them, your server contacts us 1000 times seeing if you can deliver to each address. However that’s exactly the pattern that anyone trying to do a dictionary harvest attack against us would be doing! Without special precautions, your machine will now be treated as extremely suspicious because it just tried to send to lots of invalid addresses at our server.
If you run an email server and have any more suggestions for this list, let me know at email@example.com