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Does anyone know of any studies done to show how much email will be rejected if there is not ptr record for the SMTP banner name of an email server?

Are reverse checks always done when enabled, or is it sometimes configured so if an spam program considers an email 'iffy', the reverse check is done?

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5 Answers 5

up vote 13 down vote accepted

I'd try as hard as possible to have the reverse lookup for my email server's IP address resolve to the name it's using in SMTP conversations. It makes life easier.

I don't have any statistical evidence to back this up, but it has been my experience that, from time to time, messages will be rejected by some remote mailers for not having a reverse lookup that matches the hostname used in SMTP conversations. Rather than have to deal with this problem, it's just easier to get the reverse lookup setup right to begin with. For the last few years I've just treated having a consistent forward / reverse lookup as being a requirement so that I don't have to deal with the problem of not having it.

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yup, happened to me too. Some MTAs even complained that I don't have 'mail' or 'smtp' in the server name, how weird is that. –  Aleksandar Ivanisevic Dec 9 '09 at 15:26
1  
It's a crazy, cowboy world out there re: email filtering. –  Evan Anderson Dec 9 '09 at 15:34

If you don't have it setup, you will get legit email flagged. It's up to the spam filtering device as to how they do their checks and there aren't any clear standards defined, so it's hit or miss. But in most cases with a spam score, they will ding your emails when you don't have a matching forward and reverse lookup. That ding by itself often won't get the email blocks (but no promises), but some other slight concern in the email can take it over the top.

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We didn't used to have a specific PTR record for our mail server and we were getting bounces from craigslist.org and some business partners of ours.
You also have to have a specific A record for your MX entry. No *'s or CNAMES allowed for some strict mail servers.

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When you say "banner name", it's somewhat misleading. The banner is the name on the server side, which is the side which gets to decide what to reject. :) I think you mean the name which the sending side gives in the HELO/EHLO command?

Most places just do syntax checks and reject obviously bad HELO/EHLO, because there's too much broken junk out there. But if you're running a legitimate mail-server, you should just be giving the public hostname of your own mail-server in the HELO you send out. There are some who do check. And since that hostname needs to have working reverse DNS, for your TCP connection to not be immediately rejected by many MTAs, in effect you should have working reverse DNS for the name given.

So there's no configuration hardship in getting working DNS for the HELO hostname beyond the sorting out the reverse DNS for general reachability anyway.

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As Evan said, your best bet is to have your PTR entry resolve back to your server's IP address.

However, if for some reason this is not possible, the PTR should suggest that you are using a statically allocated IP address, not a home (non-business) DSL line. As part of their anti-spam strategy, many organizations use reputation services (e.g., SORBS), which monitor IP address ranges for botnet activity and maintain blacklists of IP ranges. However, no matter of suspicious activity or not, they usually include their Dynamic User Lists (DULs) in their blacklist. To create these DULs, they use PTR records, and regular expressions, specifically tailored for the naming schemes of many large ISPs.

For example, a PTR like cable-66-103-40-69.clarenville.dyn.personainc.net would be most likely automatically blacklisted (it matches "dyn", so it must be dynamic). There are some standardization efforts on reverse DNS naming in the IETF, with mixed success:

http://tools.ietf.org/html/draft-msullivan-dnsop-generic-naming-schemes

http://tools.ietf.org/html/draft-ietf-dnsop-reverse-mapping-considerations

Slightly off topic, another source of information for black/whitelisting are WHOIS SWIP records.

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