Email Deliverability Best Practices: 4 Expert Tips to Improve Your Emails

Email Deliverability Best Practices to Make Sure Your Emails Land in the Desired Inbox

Deliverability is a key and oft ignored discipline and effector in our email communications. If mastered, we can ensure that as many as possible of our messages are reaching the greatest proportion of our desired audience, allowing our wonderfully engaging and revenue sourcing content to take over.

Ignorance on this subject, however, will lead to all of your hard work becoming a wasted endeavour as your masterpiece slips into the blackened depths of the undelivered.

To begin thinking about maximizing your deliverability, you must be capable of evolution. ISPs are investing more and more heavily in the protection of their customers’ inboxes, yet we still have little or no visibility of their cloaked, powerful and ever changing algorithms. Thus, we must invest in careful and constant monitoring, a clearly defined yet flexible strategy and a personal devotion of time to understand where the cutting edge lies and how to stay on it.

A glossary of key definitions can be found at the bottom of the page

Top Tips for Email Deliverability

IP Path

Emails are sent from an IP and that IP can only prosper with a healthy reputation. Below are some steps in the IP Path which should be adhered to to ensure that your emails have the best possible chance of inbox placement.

Quality Data Capture

This is a fundamental part of keeping your delivery rates high whilst keeping your ISPs happy and is the foundation of list hygiene. There are two different things to consider here:

Ensuring Quality Capture

This is essentially executing list hygiene measures prior to your user entering your database. There are a few options and here are some of the key ones:

  1. Email validation - Using a third party to ping the entered email address, making an SMTP connection to understand whether it is an active/live address. Note this is not always 100% accurate.
  2. Regex (regular expression) - Ensure the email address conforms to a standard structure.
  3. Common spelling mistakes; prompt users to correct upon entry.
  4. Enter email address twice on form to ensure correct input
  5. Double opt-in - confirm email address is correct from inbox. Can affect conversion rates but ensures high quality lists.

Legality of data capture and capturing engaged subscribers with managed expectations

  1. DOI (required in certain markets such as Germany).
  2. Express consent - Ticking a box to opt-into receiving emails – this can result in volume drop offs but the user expectations are much clearer, thus potentially reducing complaints and unsubscribes in the long run.
  3. Implied consent - Pre-checked boxes (i.e. users required to uncheck) and use of data set out in privacy policy and T&Cs as part of receiving a service. Facilitates maximum volume but the list quality is lower and complaints are likely to be higher.

IP Strategy

A flexible IP strategy is crucial for an email marketer to stay firmly on the cutting edge. Review your IP volumes and performance regularly and plan, particularly for your high volume sends what IPs they will be sent on in the future for consistency. Remember, a very strong IP can make a significant difference to revenue earned from high value sends.


MTD Summary

Open Rate is a very good indicator of deliverability and inbox related issues. Here there are also some ISP specific deliverability monitoring tools such as SNDS (Hotmail) and Return Path Sender Score.

Domain Analysis

In depth analysis of IP performance, with options to split by Grouped Domain and many slicers and options for other metrics.

Inbox Tools

Such as Return Path tool within Exact Target that uses a seedlist to give an indication of your inbox placement rate at all the big ISPs. Includes Campaign Overview, ISP Overviews by Country, Reputation Monitor ect

ISP Specific

Tools e.g. Mail.RU, MSN SNDS

Things to Look Out For

Sender name

  • Be transparent and avoid changing too frequently
  • Keep clarity and no deception
  • Ensure continuity with content to avoid complaints and confusion


  • There can be issues with HTML templates (30% image to text ratio is a good rule)
  • Limiting the use of images is best practice
  • Always provide alt tags and title text
  • Hosting images at the same domain as the one you’re sending from doesn’t mean you won’t have deliverability issues, it’s just best practice. It’s important that the domain isn’t blacklisted, so watch out for any third party content
  • Look out for image file size!
  • Images combined with a poor reputation could cause issues. Experts often advise that if you have deliverability issues the first thing you should do is reduce image volume


  • Despite the fact ISPs seem to be moving away from content filtering, this definitely can have a big impact.
  • In our experience we’ve seen SL tests where one word was changed and the OR doubled!
  • Be pragmatic, don’t overuse keywords! Don’t use words spammers/scammers would use.


  • Experience demonstrates that template changes can be a deliverability nightmare. Therefore they need to be designed with care.

Glossary of Key Email Terminology


Internet Service Provider (E.g. Gmail, Yahoo, Hotmail/Outlook). The domains we are sending to and the hosts of our recipients email accounts. Each has a different criteria set for reaching the inbox so you must be able to react and adapt your approach to these as separate entities.

Hard bounces

Permanent email rejection due to invalid or non-existent email address.

Soft bounces

Multifaceted, but generally this means there is a temporary reason for email rejection. For example, there is a valid email address, the message reached the mail server but the mailbox is full, the server down or the message is too large for the inbox. Also encompasses Block Bounces (complaint, blacklist, URL, content or authentication) and Technical bounces (data format, network error, server issues).


Acceptance of mail by an ISP i.e. not bounced

Delivery Rate

(Number of Emails Sent - Number of Emails Bounced) / Number of Emails Sent


Inbox placement, i.e. your message actively reached the inbox, not other folders such as Junk

Deliverability Rate

(Delivered Emails - Emails in Junk Folder) / Delivered Emails


List of entities, email addresses or domain names which are identified as trustworthy, often compiled and provided by external companies (i.e. not ISPs) but used to assist with ISP’s message blocking and filtering systems. E.g. the Return Path Whitelist


General method of blocking spam based upon the behaviour of the sending server rather than the content. This is often characterised as a temporary denial of the first sending attempt whilst the domain is authenticated, often to identify spammers. Spammers usually won’t retry to deliver the message after the temporary block in fear that this could further hurt their reputation. Reputable senders however will retry and it’s at that point that the ISP deems the mail trustworthy.


List of entities, email addresses or domain names which are identified as untrustworthy or spam. Anyone can set up and share for public use e.g. for ISPs to use as part of their filtering algorithms.

Feedback Loops

Feedback loops allow senders to receive messages back from recipients who complained. The ISP forwards the message to the sender at a designated email address, primarily so that we can remove this user from our database. Feedback loops can be a powerful tool for maintaining healthy email lists and practices.

SPF (Sender Policy Framework)

Email validation system which allows receiving mail exchangers (ISPs) to check that the mail is coming from an authorised domain.

DKIM (Domain Keys Identified Mail)

Email validation system identifying that a mail has come from an authorised domain and that the mail has not been modified during transport. The in-transit checks are the differentiator between this and SPF.

DKIM and SPF are often used to prevent forged mails and phishing by ISPs.


DOI requires an email confirmation for validation purposes. Creates very hygienic email lists but can affect conversion rates so this requires value investigation before implementation.

Email validation

On form email checks. This can take the form of basic ‘spell-checking’ e.g. checking for any known incorrect domains such as or where the email address is instantly contacted after entry to see if there is a response.

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