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Why is outbound email from my Mail Server marked as spam by Receiving Mail Servers?

It is extremely common for computer network engineers to utilize internal DNS Servers for managing private/internal processes while using external DNS for managing public/external processes on the internet.  For example, clients/servers on a private network are managed using an internal DNS while public websites on the internet are managed using the external DNS provided by a web hosting company.  It is also extremely common for customers of web hosting companies to use internal mail servers within their organizations to send outbound email.  In order for outbound email originating from an internal mail server to properly route on the internet, they must first have a Reverse DNS Entry also called an A Record for their internal mail server.  Secondly, they must have an SPF/TXT Record in order to meet the requirements of the Sender Policy Framework.

The Sender Policy Framework also called Sender ID establishes that the originating internal mail server sending the email is the authorized mail server for the sending domain.  When a receiving mail server receives an email, it performs a DNS Lookup of both the A Record and the SPF/TXT Record of the sending mail server.  It does this in order to determine the Purported Responsible Address (PRA) for the email.  The PRA determines if the received email will route to the receiving party's Inbox folder or the Spam/Junk Mail folder. When the PRA process is performed, the message header of the email is stamped with the Received-SPF value of PASS, NEUTRAL or FAIL.  A ruleset within the receiving mail server then routes the email based upon the Received-SPF value.  The majority of email service providers now utilize this process for all incoming email. 

As the likely registrar and primary DNS for your domain, your web hosting company should provide you with a Control Panel which allows you to add the appropriate DNS entries for your domain.  Without this capability in place, your email will likely continue to be marked as Spam/Junk Mail.

  • Reverse DNS Entry.  An A Record which resolves the Hostname to the IP Address of your internal mail server.

  • The SPF/TXT Record.  Example: v=spf1 ptr ip4:63.0.0.1 -all

To insure that your outbound email is not marked as spam, you may also be interested in implementing DomainKeys.  DomainKeys involves adding two TXT Records to your DNS.

Further information regarding email transport issues can be located here.

http://www.smartnetadmin.com
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