Email delivery: a complete guide

The most important step for an email campaign to be successful is - to make it to the inbox. 

Irrespective of the amount of time and effort you spend on your strategy, segmentation, and content, it all goes to waste since it never even entered the recipients’ server. Email deliverability is essentially the placement of your email in the subscriber’s inbox. However, email deliverability is dependent on your email delivery rate. Confused? Email deliverability and email delivery are not the same.  In this guide, we will cover everything about email delivery.

What is email delivery?

Email delivery refers to whether your email is received or accepted by the subscribers’ server or not. It measures the percentage of emails that didn’t bounce. Here’s how the email delivery rate is calculated:

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Basically, it answers the questions - can the email be delivered at all or not? Did the email bounce? It’s a good idea to keep track of your bounce rate to ensure a healthy email delivery rate.

 Email delivery  vs email deliverability

Email delivery and email deliverability are different things, but it’s a little confusing for most people. 

Email deliverability, also known as inbox placement, is a way to measure the number of emails that make it to the primary inbox as opposed to the spam folder. Email delivery, on the other hand, is about the first half of the email’s journey to the mailbox. Email delivery is a precursor to email deliverability. 

Let’s look at a simple analogy - you are trying to get to your office room in a secure building. You get out of your house, drive to the parking lot of this building. Then find your way, to get to the main gate of the building. You show your ID card at the entrance, and the security guard/entrance scan accepts your ID card. Then you enter the building and find your way to your room. In this case, the delivery rate would be you successfully getting into the secure building, and deliverability would be you getting into your room. 

Note: It is possible to have a good email delivery rate but poor deliverability. This happens mainly because of instances like the email reaching the spam folder instead of the primary inbox.

How does email delivery work?

The actual journey of an email from the sender to the receiver is not as simple as it looks. 

Let’s take an example. You are sending an email to me. In this case, you are the sender, and I am the receiver.

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You have typed your message and clicked send. When the email leaves your mailbox (sender’s mailbox), your email gets sent to the outgoing mail server using the SMTP (Simple Mail Transfer Protocol) server. The SMTP is a set of protocol guidelines that your message needs to follow. 

For domain verification, the SMTP server contacts the DNS (Domain Name System), which is like a phone book on the internet. It looks through the online directory to identify the sending domain and whether it can be trusted or not. After domain identification, our servers will do an authentication check of records such as SPF, DKIM, and DMARC. Many email service providers offer an in-built authentication system for setting up these records. These records help servers prevent harmful spam, forgery, and phishing.

Based on this exchange of information, my server (receiving server) will decide:

When is an email considered delivered?

Now that the SMTP server has all the required information, the email is ready to be sent to the receiver’s mail server (inbound mail server). The SMTP responds with a 250 Ok status, which indicates that the transmission has been successful, and the email lands in the receiver’s mailbox. The email is considered delivered as soon as it lands in the inbox, irrespective of which folder it goes to. A spam email is also a delivered email.

What slows down email delivery?

Email delivery is typically fast. But in some cases, delays happen - from a few minutes to a few days. There are several technician reasons behind the emails being delivered slowly:

Email greylisting

This is a method of flushing out spam. A suspicious mail is “temporarily rejected” by a mail transfer agent (MTA) using greylisting blocks. If it is a legitimate email, the originating server will attempt to deliver the mail again after a short interval. This will cause a delay in delivery. If it is spam, the delivery will not be reattempted.

Hardware error (server)

At times, the receiver’s mailbox might have insufficient storage, memory, or network bandwidth. It is also possible that the subscriber hasn’t refreshed their inbox in many days.

Server load

Between the sender and receiver, an email goes through multiple servers. If any of those servers are broken or slow, your email delivery could be delayed. These instances occur due to spam flooding, server crashes, or due to overload. Typically, mail servers keep trying to send the email until the issue is resolved.


If you were sending emails in the pre-2000s, you are likely to have heard of this. What you see now more often is ‘Mail Delivery Notification’ or something similar.

Mailer-daemon is a mail server that receives the email you send across to someone. In the event of non-delivery, the mailer-daemon bounces the email back to the sender, citing the reason for the bounce. In some cases, the mailer-daemon also encounters and sends back the email, which may appear to be spam.

How to choose the right email delivery service?

You can’t do much when it comes to email delivery, except for doing your research. Picking the right email delivery tool is the biggest determinant of email delivery. Here are some important factors you must consider: 


It is not uncommon to get emails from senders claiming to be your bank, insurance provider, or the company that you work with. It’s easy to forge emails and fall prey to phishing. Authentication protocols like SPF, DKIM, and DMARC help prove the legitimacy of your emails and help the receiving server to control spam. Check if your email service provider helps you in setting up these records for the secure sending of emails. 

Shared and dedicated IPs

Whether you need a shared or a dedicated IP address depends on the volume of emails you’ll be sending, your IP reputation, and, most importantly, your budget. Using a shared IP means your ESP puts your emails in a send stream along with other senders using the same IP address. That means, your reputation is the reputation of the shared IP address. Different internet service providers(ISPs) would look at it differently, which will impact deliverability. While this is an easier and cost-effective alternative, the risk of falling through spam filters and getting blacklisted is high.  

In case of a dedicated IP, your email delivery service will assign you a unique IP address, used by no other sender. It will put you in better control of your reputation and deliverability, but at a higher cost. Ask your ESP about their shared IP pools and pricing for both shared and dedicated IPs. 

Email Delivery rate

Now that you know what email delivery means, you ought to ask the email delivery service about their delivery rates and how they are sustaining a high delivery rate. Ensure that the tool you choose teaches you the best practices to ensure that your emails get delivered.


It is essential to go beyond surface-level metrics and access journey-specific metrics that help you make meaningful optimizations. Conventional metrics (such as open rate, click rate, CTR) often cost dearly to marketers in the sense that it is hard to establish their relationship with the bottom-line impact. However, if you have access to deeper level metrics, you can run better testing and derive actionable insights for optimization.