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 User Agent (MUA)
Mail user agent, otherwise known as email client, is a web-based or desktop application that users use to retrieve, read, send, or receive emails. They also make it possible to delete unwanted messages or flag them as spam. The most common MUAs include Google Gmail, Microsoft Outlook, and Mozilla Thunderbird, among others.

In real life, an MUA would be a local post office or mailbox to which you submit the mail you’re willing to send.

Mail Submission Agent (MSA)
Mail submission agent acts as an intermediary between the mail user agent (MUA) and the mail transfer agent (MTA). It receives the message from the email client, checks it for errors, and, if everything’s okay, transmits it to MTA.

Think of MSA as a post office counter clerk who checks whether your and your recipient’s addresses are indicated correctly on the envelope.

Mail Transfer Agent (MTA)
After receiving the message from MSA, mail transfer agent or email relay transmits messages to another computer. If the recipient’s email address is on the same server, the email will proceed to the next step in the delivery process.

However, most of the addresses aren’t. In that case, email relay or SMTP relay comes into play: MTA looks up the recipient’s domain and, if there’s a match, transfers the message to another MTA.

If we circle back to the paper mail, MTA would be the mail processing plant where the letters are sorted depending on which neighborhood they are addressed to. If the recipient’s address is in the same city, the letter will be delivered right away. If not, it might be sent to another mail processing plant or MTA closer to the recipient’s address.

MTA is also a crucial component in email queuing while sending emails in bulk. Email queuing is the process of holding off emails on the SMTP server and putting them in a sort of buffer until the recipient is ready to receive them. Email queuing is a natural process that makes it possible to prioritize certain emails (e.g., password change confirmation) over others (e.g., weekly newsletter).

Message Delivery Agent (MDA)
Once the message goes through all the MTAs, it is passed to the message delivery agent, which converts received messages into the appropriate format and transfers them to the recipient’s MUA. In the best-case scenario, messages will end up in the recipient’s inbox. However, if the sender’s domain reputation is low or the authentication fails, messages will go to the spam folder or get discarded.

MDA would be the mail carrier who picks up mail from the recipient’s post office and delivers it to their mailbox (MUA).

We hope you enjoyed reading our article. You can continue learning email architecture on the Mailtrap Blog.