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SPF Record: ISP SMTP Server

October 15th, 2009 Leave a comment Go to comments

Spam. It’s the bane of the Internet. Most of us get it. At work we get tons. Most (like about 90-95%) of the email we receive is spam. Some of it was getting through the filters too, which is even more annoying. For this reason, I’ve be doing quite a bit fine tuning of our spam filtering software, GFI MailEssentials. DNS blacklists have turned out to be the most effective, most of our spam doesn’t even get as far as the Junk box now.

Another thing I looked into was SPF records. I posted about SPF records a while back, but I thought I’d post again to give a few tips on the problems I encountered.

If you use a smarthost with your Exchange server then the server that is actually sending the emails is that of your ISP. This means that you have to put the address of your ISP’s server in your SPF record. No big deal right? Wrong.

You can enter the domain name of the ISP’s server, but if they don’t have an SPF record (which they probably wont), then yours will fail too.

It’s easy enough to find out the IP address of you ISP’s mail server (although they probably have more than one), but, and it’s a very big but, what if they change the IP address of their server without telling you? All of a sudden all your emails out would start SPF failing and possibly being marked as spam!

There is a way round it however. Using reverse DNS lookup. As long as the reverse DNS of your ISP’s mail server is set up correctly (if it isn’t, ask them to fix it) then you can enter this into your SPF. Just put this in your SPF record: ptr:mail.example.com

Done!

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