Gammadyne Corporation

Improving Email Deliverability

The following guidelines contain everything you need to know about improving email deliverability and getting your message heard.  Junk mail filters these days can be very sensitive, and it is important to know what they are looking for.

This article is primarily intended for users of Gammadyne Mailer, but most of these guidelines apply to email in general.  As always, Gammadyne reminds you not to send spam.

Content Guidelines
  • Explain to the recipient why they are receiving the email.
  • Include an opt-out link.
  • Don't spoof the From: header.  Ideally, the From: header should use the same domain as the relaying SMTP server.  In fact, SPF and DKIM won't work otherwise.
  • Don't use a free email address, like Hotmail or Gmail, in the From: header or in an email hyperlink.
  • Don't link to an IP address.
  • Personalize each email if possible.  Identical mass emails are more likely to be flagged.
  • List plenty of contact information, including your physical address.
  • Obey the CAN-SPAM Act.
  • Include a List-Unsubscribe header.  Recipients will be more likely to unsubscribe instead of filing a spam complaint.  The header should look like this:

    List-Unsubscribe: <>

  • Don't put your content in one large image.  Many email clients do not display images by default, and many spam filters don't like this.
  • The content of the email could cause it to be flagged as spam.  If it contains phrases like "buy viagra" or "penis enlargement", this is a dead give-away.  Unfortunately, the spam filter probably does not report which phrases it is flagging.  Experimentation may be required.
  • Don't use a subject that is Title Case or ALL CAPS.
  • The HTML code should be validated for correctness.  In Gammadyne Mailer, click the Validate CSS and Validate HTML buttons below the Raw HTML code.  Also, run the "Troubleshoot HTML" tool on the Test menu.
  • The Sender must be a valid email address that will readily accept email.
    • What we refer to as "the Sender" is the email address that is reported to the SMTP server with the MAIL FROM command.  Normally this is the same as the From: header.  In Gammadyne Mailer, the Sender can be specified separately from the From: header (on the Headers branch).
    • Bounce-backs are returned to the Sender.
    • Be sure to prevent this mail account from filling up.  For a large mailing, you may have to receive emails from it even as the mailing is still underway.
    • The Sender is logged within the email as a Return-Path: header.
  • There should be an empty <title></title> element inside the <head> element, despite the fact that the title is meaningless in an email.

Delivery Guidelines
  • It is okay if the email originates from a dynamic IP address, however the email must be relayed from a computer that has a static IP address (web servers have these, residences do not).
  • The SMTP relay should have a "Reverse DNS" record for its IP address.  This is also known as a PTR record.  Furthermore, the PTR record must resolve back to the IP address.  Test it here.
  • Many domains don't like it when a lot of mail arrives from one source quickly.  For mass mailings, it would be prudent to limit the delivery rate.  In Gammadyne Mailer, go to the Send/Throttles branch and enable the Universal Domain Throttle feature.  A rate of 1 email every 4 seconds is recommended as a minimum.  If time is not important, you can throttle it back even further to achieve a higher success rate.
  • If you are using Gammadyne Mailer's Direct Delivery mode, it must be installed on a web server.  Launch the "Direct Delivery Possible?" tool (on the Test menu) to verify everything is okay.
  • The SMTP relay should use TLS encryption.  This seems to improve delivery to Outlook.
  • The computer sending the email and the relaying SMTP server should have the clock set to the correct time.

Reputation Guidelines
  • Keep the mailing list clean of bad addresses.  After performing a mailing, remove all hard bounces from the mailing list.  Excessive failed delivery attempts can result in blacklisting.  For this task, it is important to use software like Gammadyne Mailer that can distinguish between a hard bounce (e.g. permanent failure) and a soft bounce (e.g. mailbox full).

    Furthermore, this is doubly important because some ISP's will reactivate old email addresses and use them as spam traps.  Once you send email to a spam trap, future emails will be blocked.
  • Use double opt-in.  This ensures that no one gets signed up without their permission, and greatly reduces the chance that you will be reported for spamming.
  • Make sure that the originating IP address is not blacklisted.  You should also test the IP address of the relaying mail server, and any sites that your email links to.  Once blacklisted, it is necessary to request removal from the organization that runs the blacklist.  To test for this, you can use this tool, or Gammadyne Mailer's "Check Blacklisting" tool.
  • Do not purchase mailing lists.  Such lists are likely to be full of bad addresses, and will generate a high rate of complaints that can result in blacklisting.
  • Do not harvest addresses from web pages.  You are likely to harvest a spam trap address.  Once you send email to a spam trap, you will be blacklisted.
  • Use Sender Policy Framework (SPF) to prevent spammers from impersonating your mail server.  Once implemented, send an email to to test it.
  • Sign your emails with DKIM.  Gmail seems to think this is important.
  • Check the reputation of the IP address for your computer and mail server with the following sites:

Deal With Complaints
  • When someone wants to opt-out of your mailing list, honor it!  Otherwise they will complain to a higher authority.
  • Create an "Abuse Address" (e.g. and monitor it for complaints.
  • Submit the Abuse Address to  This will make you seem more legitimate and allow you to deal with problems.
  • Sign up for the following Feedback Loops.  A Feedback Loop is a service that reports complaints, helping you keep your list clean and avoid blacklisting.
    Google Postmaster Tools
    Yahoo! & AOL
    United Online/Juno/Netzero