Mail rejected due to SMTP Spam Blocking: _REVERSEDNSLOOKUP, * HostKarma - Whitelist
Question asked by Jon Eastwood - 2/11/2015 at 9:48 AM
I have a number of clients who have government and Ministry of defense clients who keep getting email bounce by our servers for spam filtering with the error:
Mail rejected due to SMTP Spam Blocking: _REVERSEDNSLOOKUP, * HostKarma - Whitelist
I am sure the MOD and governments have their servers set correctly so does that men I have might setup wrong?
Based on various recommendations by SM and other people I have my reverse DNS weight score set to 35 and my SMTP Blocking weight threshold set to 25, so I can see why the above rule is firing.
HostKarma - Whitelist is set to score 0
Are these scores TO STRICT or should I up them?

12 Replies

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Joe Wolf Replied
Post one of the IP Addresses involved and we can test and find out the answer.
Jon Eastwood Replied and email is coming from ?????@caa.co.uk
Employee Replied
Employee Post
Hello Jon,
Thanks for the information. Sounds like the Spam weights threshold was met and based upon the failed _REVERSEDNSLOOKUP spam check the email was rejected. What do you have for the Spam filters?
Click on the Security Shield | AntiSpam Administration | Filtering. What do you have for the low, medium, and high weights?
Please provide us more of the log entry and we can look at the full spam checks.  If you prefer to open a Support ticket for more review please do so at your convenience.
Steve Reid Replied
Having smtp blocking base on one test alone is not a good idea. I have mine setup in a way that it usually needs to hit three or four failed tests before it blocks them.
Also just because they are the MOD and government does not mean they have their server setup properly. If they fail that test when you manually check them on mxtoolbox.com then Smartermail is working right.
Jon Eastwood Replied
Hi Steve, I think looking further as I said the DNS score for a failed reverse dns look up was set at 35 which i think is too high, whats other people have that test set to as a weight?
Steve Reid Replied
I have it set the same weight but I have SMTP blocking set to 75
Jon Eastwood Replied
ok, cheers
i got the same issue. i set Low=10 Medium=15 and High=50.

Please advise
Hi Steve. what do u mean on SMTP blocking set to 75? is it the Imcoming Weight Threshold?
David Finley Replied
I noticed that one connection was blocked on our smarter mail server, but when I looked at it in MXtoolbox, the server correctly responded with a reverse DNS.  Any idea how smarter mail could fail something that probably should not be listed as not complying with reverse DNS?
Bruce Barnes Replied
While Steve is, technically, correct in the fact that you should not be basing e-mail rejecting on a single test, be advised that Outlook.com (and domains hosted by Outlook.com; Yahoo.com (and domains hosted by Yahoo.com; Hotmail; Google.com, and domains hosted by Google.com; and AOL; will no longer accept e-mail which does not an a PTR or rDNS entry setup in the DNS for the domain.
Here's a link to an article which clarifies this, along with a couple of other points: 

Over the past few years, the exponential increase in the volume of spam has forced email service providers to apply increasingly aggressive filtering solutions which sometimes exclude legitimate emails. In order to reduce the chances that your emails are perceived as spam from major suppliers such as Yahoo, Hotmail, Google or AOL, please make sure your server meets the following qualifications :

1) A valid “PTR” Record (Reverse-DNS) is configured on your IP address.

A PTR Record is essentially the hostname that is shown when a reverse lookup query is done on an IP address. This type of query is always done by default and the result is shown in email headers. Having a configured or customized PTR record will help giving your server more credibility when sending emails.

You will also need ABUSE@DOMAIN.TLD and POSTMASTER.TLD for EACH hosted domain.

An optimal PTR record would be the server’s principal hosted domain.

You can have your Reverse DNS configured or changed by opening an assistance request with our system administration team.

2) The email addresses: abuse@.tld, postmaster@.tld should be working.

Feedback loop is an email address used to report spam abuses for a certain IP address. This address is abuse@ and/or postmaster@. AOL asks that these account exists and that they accepts emails.

3) A “TXT” DNS entry that contains a valid SPF record is configured for the domains hosted on your server.

By default, anyone can send emails from any domains by spoofing email addresses. A SPF record will restrict the IP addresses or hostnames allowed to send e-mails from the domains it will be configured on.

You can generate a SPF record by using the OpenSPF Wizard.

By having the two conditions above met, the verifications made by the email service providers are likely to determine that your IP address can be trusted and will refer to other criteria to verify wether a specific email is considered unsolicited.

If you wish to know more about how email delivery works and ways to raise your delivery rate, you can review the following website: http://www.saas.nsw.edu.au/solutions/dns.html

Do not hesitate to contact our team if you have any questions or require assistance.

Bruce Barnes
ChicagoNetTech Inc

Phonr: (773) 491-9019
Phone: (224) 444-0169

E-Mail and DNS Security Specialist
Network Security Specialist

Customer Service Portal: https://portal.chicagonettech.com
Website: https://www.ChicagoNetTech.com
Security Blog: http://networkbastion.blogspot.com/

Web and E-Mail Hosting, E-Mail Security and Consulting
David Finley Replied
Thanks Bruce good information as always.

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