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An ultimate guide to email infrastructure

While sending emails is a straightforward process for individuals, organizations don’t have that luxury. They must put a lot of thought into this process to avoid security threats and ensure all of their emails get to recipients’ inboxes. That’s where email infrastructure comes into play. 

Though it operates behind the scenes, it combines all the components that are responsible for sending and delivering emails. Today, we’ll break down what email infrastructure is, how it works, and what can be done to boost its performance. 

What is email infrastructure? 

Email infrastructure is the set of software and hardware components that are triggered as soon as you write an email and hit the send button. It combines mail servers, agents, and IP addresses – basically everything that you’d need for the successful delivery of email campaigns.

Think of it as a postal system that combines postal offices, staff who sort through mail, in-house or third-party delivery services, and postal carriers who bring mail to your doorstep. 

Even though emails are written and sent through the internet, the structure of the infrastructure is rather similar to real-life postal services (excluding all the servers and authentication protocols, of course). 

Why would you care about email infrastructure? 

Let’s say you have a paper invitation to deliver to your colleague. You drop the letter in the mailbox, but it doesn’t get picked up on time. While you’re waiting for the mail carrier, someone breaks into the mailbox, changes the letter’s content, and disappears. 

The letter gets delivered to the recipient eventually, but instead of containing an invitation to the New Year’s Party, it asks your colleague to donate money to the swindler’s bank account. 

That’s exactly how phishing attacks are executed in the world of electronic mail. Weak email infrastructure becomes an easy target, allowing attackers to hijack reputable email addresses, forge the content of emails, and deceive recipients. Such attacks are not only harmful for the recipient, but they also have dire consequences in terms of data privacy and protection. 

Another point to remember here is that poorly-executed email infrastructure can have a direct impact on sender reputation, causing the recipient’s email client to think that your domain isn’t secure and mark it as spam, reducing the deliverability rate of your transactional emails. 

Email infrastructure architecture

Email infrastructure consists of the following elements: mail agents, mail servers, authentication protocols, IP addresses, and feedback loops.

Mail agents

Mail agents are vital components of email infrastructure as they take care of the whole process from sending to transferring to delivering the emails and showing them to the recipient. Typically, four types of mail agents are used to send emails: MUA, MSA, MTA, and MDA. 

Mail servers

It’s virtually impossible to send an email without email servers. There’s only one outgoing server called SMTP server, and two inbound servers: IMAP and POP3. 

SMTP server receives the messages from MUA and conducts authentication checks through DNS records to verify the recipient’s email address. If any of the checks fail, the email will bounce back or end up on the blacklist (if the email reputation is low).

If the authentication is successful, emails are delivered through mail agents to the recipient’s inbound mail server, either IMAP or POP3, depending on which one is set up. 

IMAP stores received messages on the server and downloads them once the recipient retrieves them from the inbox, saving the copy within the server. This is the option most email clients incorporate into their email infrastructure. 

POP3 also stores messages on its servers, but it deletes them as soon as the recipient retrieves their emails. It’s less dependent on an internet connection since the messages are downloaded at once and can be accessed even offline.

IP address

An IP address, otherwise known as an Internet Protocol address, is a combination of numbers (or numbers and symbols in the case of new addresses) that locates and identifies nodes within the computer. It’s a numerical address of the domain, determined by Domain Name System (DNS). 

Thanks to DNS, alphabetical names of domains (,, etc.) can be translated into numerical IP addresses ( in seconds. IPs are essential for communication and data sharing between networks and computers i.e. they are vital to internet operation.

There are two main types of IPs: shared and dedicated.

Authentication protocols 

Email authentication protocols are ‘defense mechanisms’ used by the recipients’ servers. They ensure that the message wasn’t hijacked, altered, or forged before getting delivered. To validate the accuracy and authenticity, most modern servers use the following protocols: 

  • SPF: a basic protocol that is used to check if the sender’s IP is authorized. SPF has a TXT format and is stored in DNS records. The recipient’s server will scan DNS to check if the IP belongs to the list of authorized IPs. If so, the email will be delivered. If the authentication fails, the message will be either flagged or rejected. 
  • DKIM: a type of digital signature that is included in the email’s source code to check if the sender’s domain is authorized and verify that the contents of the email weren’t altered. DKIM utilizes a set of tags and characters to create a hashed private key. Once the email gets to the receiving server, it searches for the corresponding public key. If there’s a match between public and private keys, the authentication will be successful. 
  • DMARC: a more advanced authentication method that utilizes either or both SPF and DKIM. While setting up DMARC, the domain owner can choose which of these protocols will be used for authentication and how ‘strict’ the validation process will be (whether the domains should be the exact match or subdomains are also allowed). DMARC protocol enables domain owners to improve their sender reputation gradually by instructing the receiving server on what to do if authentication fails (quarantine, reject, or nothing). It sends reports of each failed authentication and allows for a quick response in case the sender domain gets spoofed. 
  • BIMI: the newest of the authentication protocols that uses DMARC but adds another layer of security by validating brands’ logos. If BIMI is set up, DNS will include the TXT record with information about the brand’s logo. The recipient will see that logo when they receive the email from that brand. BIMI isn’t widely adopted yet as it’s still being tested by email providers. 

Feedback loop

Email feedback loop (FBL) is another important component of email infrastructure. It enables organizations to receive data about users’ spam complaints from email service providers (ESPs). 

By doing so, senders get the chance to remove clients who are no longer interested in their content from their email list. Spam complaints are a nightmare for any email marketing campaign as they affect sender reputation and cause deliverability issues. 

However, not every user is eligible for FBL registration. It’s not provided by every Internet Service Provider (ISP) either. To qualify for FBL, business owners should satisfy the following criteria: 

  • Be the owner of the IP or domain or have admin rights; 
  • The Reverse DNS of the IP should be the same as the one you’re using;
  • Have a functional postmaster@ or abuse@ email address for the domain for reporting spam or errors; 
  • Have a good reputation (can be observed and maintained with the help of reputation management tools). 

What are the types of email infrastructure, and how to choose the most appropriate one?

The most common types of email infrastructure are cloud-based, on-premise, and hybrid:

  • Cloud-based or hosted email infrastructure means that the entire email infrastructure operates on the cloud with dedicated sending and receiving servers. It makes it possible to easily manage the users, take care of the maintenance, use provided technical support, incorporate additional features, and access the data through existing email service providers; 
  • On-premise MTA or commercially-licensed email infrastructure is a secure hardware solution that provides complete control over email infrastructure design, sending threshold, campaign tracking, security, data privacy, and so forth; 
  • Hybrid email infrastructure combines the features of on-premises and cloud-based infrastructure. It uses on-site hardware, while also storing data and operating on the cloud.

Thanks for reading our guide. If you want to know more about the components of email infrastructure, you can visit the originally published guide on Mailtrap Blog.

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