Introduction to Deliverability

  • Updated

This section provides basic information and best practices for a successful deliverability.

Email can be personalized, automated, tested, measured and more. Email marketing is still one of the most effective options to marketers, especially when they tap into the best practices for enhancing deliverability.

Delivery vs. deliverability

Email delivery refers to the successful delivery of an email to the receiving server.

Email deliverability refers to the ability to deliver email to the intended recipient’s inbox.

It’s possible to have good delivery but poor inbox placement, because the email lands in the spam folder rather than the inbox.

Key figures of deliverability

An email is bounced when the receiving server, mostly the ISP, rejects it. Based on the error message the receiving server sends back, the rejection can either be a soft bounce, which is a temporary error, such as mailbox full, spam related, or time out; or a hard bounce, which is a permanent error, such as the email address is no longer valid.

Optimizely Campaign has an overall delivery rate of 99.8%. Without knowing what percentage of emails arrives in the inbox and what percentage in the spam folder, Optimizely can make inferences by analyzing key metrics for the emails.

The following measures give marketers insight about the reaction and engagement of their audience.

  • Bounce rate


    		Bounces=    -------------------------  x 100%       Number of sent emails

    Depending on the business model, it is recommend to keep the bounce rate under 3%.

  • Open rate


    	  Number of opened emails=    -------------------------------  x 100%	Number of delivered emails
  • Unsubscribe rate


    	   Number of unsubscribes=    -------------------------------  x 100%	Number of delivered emails

    Generally, an unsubscribe rate below 0.5% is good for an email campaign.

  • Click-trough rate


    	  Number of click-throughs=    -------------------------------  x 100%	Number of delivered emails

    An email click-through is defined as the number of recipients who click links in an email and land on the sender's website, blog, or other desired destination.

  • Spam complaints: Reports made by email recipients against emails they do not want in their inbox.
  • Spam trap hits
    • Recycled spam traps are email addresses that were used by a person and then abandoned. Typically, if an email address has been dormant for the last year, many ISPs convert it into a spam trap.

      Sending to recycled spam traps shows that you have poor list hygiene, or you are not removing unengaged users.

    • Pristine spam traps are set up by ISPs and anti-spam organizations in order to catch spammers. No one should be sending email to those addresses.

Marketing opt-in

There is a big difference between having a list of email addresses and having permission to email to that list. There are three types of permissions:

  1. Single Opt-in (SOI): the interested party leaves his email address in a form, which is considered explicit consent to receive email newsletters regularly.
  2. Confirmed Opt-in (COI): after a single opt-in, an email is sent to subscriber’s email address as confirmation of the registration.
  3. Double Opt-in (DOI): after single opt-in, an email with a confirmation link is sent. To complete the subscription process, the subscriber must verify the registration by clicking the link.

Sending email to non-permission-based lists results in high bounce rates, low open rates, high unsubscribe rates, high spam complaints and high spam trap hits, all of which indicate an unsuccessful email.

For more information on the opt-in method, see Opt-in.

Sender reputation

Having the ideal ratio of text to images, considering the placement of the content elements, using an eye-catching font, personalizing the content for the target audience, and selecting the optimum sending frequency are all key factors in a successful email. However, all that effort is wasted if the intended audience never opens the email because it does not reach their inbox.

Filtering mechanisms are becoming increasingly sophisticated in determining if an email is important, junk or spam. Some check the content, while others focus on the recipients’ engagement. However, from an ISP’s perspective, the sender’s reputation plays a greater role in the analysis.

Here are typical factors that determine a sender’s reputation:

  • Total number of emails sent
  • Number of emails sent to invalid email addresses
  • Number of recipients who mark the emails as spam
  • Domain from which the emails are sent
  • Whether this domain has authentication records
  • IP address from which the emails are sent
  • whether the IP address is blocked anywhere
  • whether the IP address is dedicated
  • whether the IP address has authentication records

Email authentication

Authentication allows the mailbox provider to confirm that the sender is the one who he pretends to be. There are four primary methods of authentication:

  1. Reverse DNS, which implies determining which host and domain name belong to a given IP address. If a Reverse DNS Lookup returns a no domain associated, the email will likely bounce to the sender, or be deleted or filtered. This entry is set by default on the Optimizely sending infrastructure.
  2. SPF is Sender Policy Framework, which states which IPs are authorized to send on behalf of the "From" domain. SPF allows the receiver's host to verify that the email is being sent from the server it asserts it is sent from.
  3. DKIM is Domain Keys Identified Mail. The recipient uses this to determine that the message has not been altered in transmission. The public and private keys must match to ensure that nothing happened to the message in transit.
  4. DMARC is Domain-Based Message Authentication, Reporting, and Conformance. DMARC ensures that the legitimate email is properly authenticating against established DKIM and SPF standards, and that fraudulent activity appearing to come from domains under the organization’s control (active sending domains, non-sending domains, and defensively registered domains) is blocked.

List management and hygiene

Marketers must ensure their emails are sent to an interested audience and, at the same time, care about the audience experience.

It’s important to nurture new subscribers and make them feel welcome, while also rewarding active subscribers and maintaining their interest.

It’s also important to regularly check in with less active subscribers and give them the opportunity to re-engage or opt out. If left unchecked, the number of inactive people will grow. Continuing to send emails to them will negatively affect the sender’s reputation and impact deliverability, even to active subscribers.

So, the best way to segment the recipient list is by engagement level.

  • Sending to highly engaged recipients frequently, even on a daily base.
  • Sending less frequently to less engaged recipients.
  • Using reactivation campaigns to recipients who have not been engaged for 3-6 months.
  • Blocklisting recipients who complained that the email was spam.
  • Deactivating recipients who did not engage within a certain amount of time (1 year, 20 months and so on). Do this once a year.

Constantly monitoring the activity and engagement level of subscribers allows marketers to better understand what the subscriber wants and tailor emails to cater to their needs.

For more information and best practices, see: