Email Deliverability

Email deliverability is the measure of likelihood that your email will reach your customers' or subscribers' inboxes. 

ESPs such as Google, Hotmail & Yahoo all employ anti-spam techniques and algorithms to detect and isolate spam messages. These techniques & algorithms are updated very frequently and are not made public, so it can be difficult to determine exactly what in your email triggered a spam warning or a content block. Below are some of the universally accepted methods of staying on the good side of ESP spam detection and ensuring inbox placement.



Each domain (e.g. has a domain management system, in which Admins can control settings and configuration for that domain. When sending Email, the domain you are sending from should have email authentication configured in the DNS for that domain. The methods of authentication provided by CustomerMinds are:

  • SPF (Sender Policy Framework) tells an ESP that the Which50 mail server has been authorised by your domain’s administrators to send email on your behalf. This is a one-line TXT record in the DNS.
  • DKIM (DomainKeys Identified Mail) digitally signs each email sent from your domain. The digital signature is not visible to end users but allows an ESP to look up this digital signature and match it to a paired public key in your domain’s DNS. If matched, the ESP knows the email is authorised and legitimate.

Our team will provide the necessary documents and instructions to correctly configure your domain. This is usually done when your account is initially set up, but over time, branding and domain changes may require updates to the authentication. You can check this with us any time - just email and we'll be happy to take a look.

IP Address Reputation

If a new IP address is set up, it must be “warmed up” before sending large volumes of email. This is done by starting with low volumes and increasing gradually. Over time, ESPs start to trust the IP address and increase its reputation. An IP address’ reputation can decrease if large volumes of email start going to spam (i.e. if they are unauthenticated & contain questionable content), or if a lot of spam reports are made. Which50 sends through IP addresses which are well-established and maintain a good sender reputation.


Human Interaction

Humans can have positive and negative impacts on inbox placement – a higher level of engagement through opens and clicks results in a diminished chance of ending up in spam. Conversely, the more spam reports an ESP receives for emails sent from your domain, the greater the risk of being sent to the spam folder.



Which50 has native throttling built-in, which distributes email volume across different ESPs, so they are not overwhelmed with large volumes. However, when you have a large-volume send or an anticipated response (e.g. calls to contact centre), it’s best to include throttling on the email activity in Which50 too.

We recommend throttling on an hourly basis to give the most control over the rate of send.



If your email is authenticated correctly, your IP reputation is good, and you have a high level of engagement on your emails, then you have more freedom when it comes to your content. However, if all the above is not in order, then you have a greater risk of ESPs paying more attention to the content of your subject line, email links and email copy.

ESPs will look at the size of your email and will also run keyword filters using the most common words and phrases found in real spam emails. They will also check your links and any attachments.

  • Email Size
    • 100kb is a good limit to keep in mind for emails – this includes the HTML and images, so it’s a good idea to resize and optimise all images. Emails over this size will take longer to download and are more likely to be sent to spam. Which50 displays the current size of your email above the thumbnail.
    • It’s also better to keep your email as short as possible – long emails with lots of text can look suspicious to ESPs, as can emails with lots of images. Use a landing page if you need to provide a lot of information with the email.
    • Attachments will obviously bump up the overall size of the message, so ensure they are optimised as much as possible and use the throttling feature in Which50 to manage the pace of your email send.
  • Spam trigger keywords are usually predictable, ranging from the obvious inappropriate language to the downright mundane, such as “free”, money”, or “win”. Search "Spam Filter Keywords" to find the most up-to-date lists. Obviously these are all based on observation, and you’ll find many blog posts on the subject. ESPs do not provide lists of keywords they filter on, and these lists evolve over time as well. As you can imagine, it’s probably downright impossible to avoid all spam trigger words completely, so it’s important that you ensure authentication and reputation are in order, so content doesn’t become an issue.
  • Links should be tested before any emails are sent – it’s important to ensure that there are no security warnings on the pages you are linking to – i.e. no browser warnings pop up, and if your link includes the https prefix, it must have a valid SSL certificate.
  • Attachments should be optimised as much as possible to reduce file size, and only used where necessary (usually due to a compliance requirement). The better alternative is to host the PDF either on your server or via Which50, and link directly to the document for download – this method avoids the attachment increasing the overall email message size, and therefore reduces the likelihood of being caught in a spam filter.


Contact Lists

The usual due diligence should be conducted when pulling data for a campaign – confirm the provenance, quality and consent status (if marketing) of the data before you upload to Which50. Which50 will do some validation on upload, unsubscribing any email addresses which are not in the right format. Maintaining a clean database of email addresses will help your email to land in the inbox in a few ways:

  • Spam traps – old email addresses are sometimes used by ESPs to identify organisations who send bulk mail indiscriminately. If you are often mailing these non-engaging email addresses, it increases the chance of your emails going to spam. Using Which50 reports and contact audit tools will help you to identify email addresses which are not engaging, and you can remove these from your contact lists.
  • Bounces – similarly, if ESPs find they send a lot of bounce notifications back to your domain (via Which50), it indicates that your contact list is not up to date and they may start redirecting your emails to spam. Which50 automatically unsubscribes any email addresses which return a hard bounce, or soft bounces from three separate emails (this limit can be customised), thus keeping the list clean.
  • Engagement – cleansing bounced emails and non-engaging emails from your lists will result in a higher rate of engagement per campaign, which links to point 3 above.



In summary – there are many different factors at play when ESPs look to determine what is legitimate email and what is spam. By following best practice and ensuring all your email domains are correctly configured with the right authentication settings, you can reduce the risk of spam placement greatly. However, ESPs change their methods regularly and often without notice, so it is a useful topic to stay up to date on.


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