What is Email Deliverability: A Complete Guide

November 10, 2023

Email Warm Up & Email Deliverability

Sender Score & Understanding Reputation 

Understanding email sending and your underlying email reputation is complex and comprises different reputations that ESPs (email service providers) and ISP (internet service providers) use to determine overall email delivery. Depending on the ESP you use they will rank these reputations differently. For example, when sending to Outlook accounts, Microsoft will favor technical setup and authentication over domain reputation whereas Google (G Suite) is known to favor domain reputation and email behavior over technical authentication. Nonetheless every factor within your control in your email program is important and should be monitored. 

Before jumping in my disclaimer is as follows: 

Email deliverability is by design uncontrollable, only influenceable. Therefore, it is not possible to make claims or guarantees on inboxing. Everything that follows is what we know to be best practice but subject to change as the policies and techniques used in email evolve. 


Email has evolved considerably over the years and I’ll provide some short examples as to WHY they are considered important. Note that these reputations (see graph above) are in place to combat spam email which unfortunately still represent the majority of emails sent today. 

Before jumping into the factors that we can directly influence, here is a handy graphic for understanding the email flow from sender → receiver. Read the full flow walkthrough here

The takeaway here is that there are some sections that are not controllable by us (the sender) and dependant on the ISP or ESP you are using. What this means in practical terms is that just because your email landed in spam for Google it does not mean that it landed in spam for Outlook. Everything in the ‘ISP decision’ quadrant is not controllable by us. For example, even if you have the best email configuration and email program setup in the country, if I as an email receiver set up as a rule to not allow anybody with the letter ‘u’ in the domain name to reach my users inbox there is nothing as a sender that you can do. 



  1. IP Reputation
    IP Reputation indicates how much users want to get email from a given IP address by measuring bounces, spam or unwanted bulk mail.

    Overtime, measuring only a sender's IP reputation would prove to be problematic as a malicious sender would only need to ‘swap’ the sending IPs if the original IPs being used were to be flagged as bad.

    In most email setups you will never need to monitor the health  of your sending IP. Any mainstream email platform (mailchimp) or provider (Gsuite) will provide the IPs and monitor the health. The only exception is on higher tier plans on automation platforms (MA) such as Sendgrid where you can purchase a dedicated IP for sending. Why you would want to have a dedicated IP is out of scope for our purposes. In our context, G Suite manages the IPs and their reputation. You can actually look up any IP address to know their reputation by going here: Cisco Talos Intelligence Group. By searching for an IP it will tell you the owner, sending history and it’s ranking (Positive, neutral, Negative/Spam). Achieving positive ratings is done through sending email ‘that is wanted’ that is, recipients do not mark it as spam and respond to it.

    If you were to be sending suspicious email content through Gsuite for example they are known to have different tiers in their IPs each with different scores. As you improve your sender score so will the pool of IPs you are able to send from. The better the IPs reputation the greater the chances of landing in inbox and not in spam.

 2. Content Reputation
Content reputation works on a set of criteria that determine the sender’s quality of their email campaign content. While certain types of content are clear triggers for ISPs’ content filters (attaching a virus, a string of words asking for bank details, and so on), a sender’s content reputation goes down when their emails keep getting low open rates or are flagged, blocked, and unsubscribed.

IP and content reputation work hand in hand to create an overall picture of a sender’s email practices. IP reputation determines the quality of a sender’s email sending through their emailing history. Content reputation analyzes the type of content a sender’s email has and determines if the sender is trustworthy or not.

One question you may be having is ‘why are links so sensitive in email?’. The answer here is a piece of history. In the early days, scammers would send emails that asked for banking info or telling the recipient that they had won a sum of money. ESPs were able to combat this by reading the email contents and knowing that if the email contained a certain amount of spam words (List of Known Spam Words). Spammers fought back by taking all the text and putting it on an image and including the image as the only ‘text’ in the email body. At the time, computers were not able to understand what was contained in the image and was therefore unable to know if it was a picture of your dog, or a wall of text.

**The response was to treat emails with low text to image ratios as inherently suspicious.**

It’s for this reason that many email experts will suggest disabling link tracking and open tracking when sending short emails since open trackers are technically images that act as pixels.

Example snippet from mailchimp below:

Example snippet from email on acid below:

Notice how they are different ratio suggestions from each? Frustrating right?! This is the world of email where nothing is known with 100% certainty. However, there are general guidelines that we are aware of that adhere to.

I include this to illustrate that nothing written here is absolute, email is constantly changing and each ESP will look at the problem of triaging email differently in an attempt to protect their users inboxes from harmful content (whatever their definition of that is!). If they were to vocalize how to get past spam filters and help in any way for deliverability this would unfortunately largely only benefit spammers.  

To learn about how we approach copywriting visit the process document: Ubico - Copywriting Process

  1. Domain Reputation

    As authentication systems have become more robust, ESPs have developed a metric called domain reputation, which measures the quality of a domain’s authenticated emails. Domains and IPs can be different, after all. For example, Mailchimp customers are likely using shared IPs sending and sending through various domains.

Email sending reputation is a complex metric of other different reputations to determine email delivery practices. It’s developed, essentially, through a constant game of chase-and-catch between hackers who send malicious spam and the ESPs that are constantly creating new ways to catch them in the act.

Great email sending practices do not end with the way you create the content and design of your emails, but they also depend on following strict security protocols that help ISPs identify you as a trustworthy sender.

My take is that domain reputation is the most ‘black box’ out of all the reputations since it largely has to do with email behaviour and how the recipients interact with your emails which is information that is largely hidden from you unless you have certain tools setup to track, however even with these tools it is known to be unreliable and you need to be sending a certain volume of emails to even have a benchmark. 

Bad practices and setup usually lead to your domain getting ‘blacklisted’ or ‘burned’. This is usually the result of a bad sender score and sending habits.

Read more about sender score here and known pitfalls.


Now that we have an understanding of the various elements that must be considered when building an email program let's look at how to set this up for our purposes. I will be referring to the sections above often as they are the building blocks of email and all equally important to get right. The goal is to improve our sending score as much as possible for the highest chances of deliverability. 

Technical configuration

Authenticating your account ensures that only a specific list of IPs can send emails using your domain. This keeps spammers from falsely delivering emails through your domain.

Think of DKIM as the signature you include in every email campaign. The DKIM is a powerful proof that the recipient’s ESP can use to check if the emails they have received are domain-authenticated and valid. So, if the signature matches, then the email goes into the inbox – all other things equal. If it does not match, then it’ll go into the spam folder (or get a hard bounce).

SPF, DKIM are the minimum for email authentication today. See below for setting up with Gmail.

  1. Setup SPF

        2. Setup DKIM 

         3. Conduct Mail-tester test 

  • Newsletters spam test by mail-tester.com
  • Use the EXACT subject and message you would be sending as an example through outbound. (If you can send directly from the email automation service even better since it will include tracking pixels which wont be present if sent directly from Gmail client. 
  • Look for any negative points being removed due to authentication or links being broken. The score below was sent using our primary ‘ubico.io’ domain and is fully authenticated with DKIM, SPF and DMARC. From a technical perspective the domain is properly configured. 

    Note: **A perfect score on email sending DOES NOT correlate with 100% inbox placement. This tool is purely for testing the authentication and technical setup of the sending address. ‘Where’ your email will land (inbox/spam) is a result of many factors including: sender reputation, volume, history and content.**

Email Warmup

There are two ways of ‘warming up an email account’, the first is manual, the second automated. 

**Note: when we refer to ‘warming up an account’ what we are really discussing is how to simulate or spoof a real email account for the purpose of only sending outbound emails. Generally, this is not required if you are using your primary domain and are using your account for general email purposes such as emailing coworkers, friends, suppliers and clients** 

Note: Before doing any email warmup its best practice to wait 14-30 days MINIMUM before doing any sending with a new domain. By default all newly purchased email domains are added to all ESP blacklists to prevent spammers from buying new domains and burning through them. 

Manual warming up of an account is not recommended as virtually all experiments in the past have led to inconsistent results in delivery overtime. Simply put, scaling manual sending behavior and reputation management is incredibly difficult. 

Therefore, the best method of simulating an account for outbound purposes is to use a warm up service. All warm up services have the same underlying principle of generating ‘positive interactions’ with your emails to increase your sending reputation. As we learned above, this directly ties into your domain reputation which is the leading factor for deciding your sender score. 

How do these services generate these interactions? Simple - they own a seed network of email accounts from ALL major ESPs that they control. Yes, this means they literally own thousands of gmail, outlook, yahoo and other email accounts that will automatically respond, star and never move your messages to spam. They increase the throughput of interactions needed based on your sending volume. The cost for these services increases as the number of interactions are needed and the quality of the seedlist. This continuous feedback of positive interactions from all ESPs will artificially increase your sender score and allow you to send outbound emails more predictably. 

Reminder on deliverability: 

If you recall from above, we know that based on our reputation we will be placed in different sending tiers within our email provider, therefore, by having ‘guaranteed’ engagement we also benefit from better sending IPs which leads to higher chances of inbox placement. Obviously, authentication must be taken care of beforehand however automatic warm up will help with IP reputation, domain reputation as well as our overall sender score. Not bad! 

If this sounds too good to be true, you are correct - ultimately, this service will help however it does not replace other factors such as: 

  1. Email content (is it personalized? Do I have a million links?)
  2. Email sending volume (did I send 500 of the same email with no personalization in the same day?)
  3. Recipient behavior (was my targeting correct? If its not they will likely mark it as spam)
  4. Authentication (Do I have my security set up properly?)

If your complaint rate (people that mark the email as spam) with Google for example is high (say above 1%) you will still have difficulty inboxing to google users since your emails are still generally seen as not wanted. 

This is but one piece of the puzzle towards striving for perfect inboxing at scale. Although expensive I have yet to find a better way of not burning through alternate domains. 

Remember, the goal is to be able to reliably send emails from alternate domains overtime without compromising on inboxing. 

Some email warmup services: 

  1. inbox ignite 
  2. Spamtoinbox 
  3. Lemwarm (community feature)  
  4. Mailwarm  

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