What is an Email Server and How Does It Work?
An email server or mail server is a critical part of email infrastructure, handling the transmission and storage of emails. If you’ve ever wondered how an email travels from your computer to your recipient’s computer, it happens through email servers.
The way email servers function is highly technical, so we made a special effort to put together a simple guide explaining how they work.
What is an Email Server?
An email server is a computer system that sends, receives, and stores email messages. It functions as a digital post office, handling the delivery and management of electronic mail.
A mail server uses a combination of software, hardware, and protocols to send, receive, store, and manage emails:
- Mail server hardware: The hardware refers to the physical computer systems that house the email server software. These powerful computers have ample storage space and network connectivity to handle the large volume of emails being sent and received.
- Mail server software: This is the mail server program itself that runs on the hardware. The software is responsible for functionalities such as receiving emails, storing them, routing them to the correct recipients, and implementing security measures.
- Mail protocols: Email protocols are standardized sets of software rules that govern how email messages are transmitted, received, and stored across networks.
Email servers do much more than send and receive emails; the next section provides more insight into their tasks.
What Does an Email Server Do?
An email server takes on a range of duties; here’s what it does on a typical day:
- Sends emails: When a user sends an email, the email client communicates with the mail server using SMTP (Simple Mail Transfer Protocol). The server then forwards the email to the recipient’s email server.
- Receives emails: The recipient’s mail server accepts incoming emails from other servers and stores them until the recipient retrieves them using an email client.
- Stores emails: The mail server stores incoming emails in the recipient’s mailbox until they are accessed. It also keeps a copy of sent emails in the sender’s outbox if configured to do so.
- Manages user accounts: Mail servers manage user accounts, including authentication, storage quotas, and configuration settings.
- Security and spam filtering: The server scans incoming and outgoing emails for spam and malware, using various filtering techniques to protect users from harmful content.
- Encryption: Ensures secure email communications by implementing encryption protocols such as SSL/TLS for data transmission.
- Handles email delivery failures: Manages emails that cannot be delivered to the recipient, sending bounce-back messages to the sender with details on why the delivery failed.
- Resends emails: Attempts to resend failed emails according to retry policies before declaring them undeliverable.
- Logs activities: Keeps logs of all email server activities, including sent, received, and failed emails, for troubleshooting and auditing purposes.
- Monitors performance: Continuously monitors the server’s performance, availability, and security to ensure smooth operation and quick response to any issues.
Different components of mail servers team up to complete these tasks — the next section breaks them down.
The Components of an Email Server
Just as there are layers in network architecture, mail servers consist of components that help it work smoothly.
All mail servers consist of these parts:
| Email server elements | What do they do? |
|---|---|
| Mail Transfer Agent (MTA) | Responsible for transferring email messages from one server to another. It uses protocols such as SMTP to route emails. |
| Mail Delivery Agent (MDA) | Once the MTA receives an email, the MDA delivers it to the recipient’s mailbox. The MDA works with the email storage system to ensure the email is properly stored. |
| Mail User Agent (MUA) | This is the software used by the end-user to read, send, and manage emails; it’s better known as an email client. |
The above table explains the conceptual structure of email server software. Regardless of the operating system, MTAs, MDAs, and MUAs are critical in email delivery.
Unix, Windows, and Cloud-Based Mail Servers
Most mail servers today fall under these three types:
- Unix mail servers,
- Windows mail servers,
- Cloud-based mail servers.
There are also open-source email servers, but they aren’t too common.
Unix Mail Servers
These servers run on Unix-based operating systems such as Linux or FreeBSD. They often use a modular approach with separate software components for different functionalities (such as Sendmail for sending emails or Postfix for mailboxes). This methodology allows for flexibility and customization.
However, Unix servers require more technical expertise to set up, configure, and maintain due to the modular design and need for command-line interaction.
As for email server security, it depends on the administrator. Regular updates, secure configurations, and potential additional security software are necessary for Unix servers.
Windows Mail Servers
These servers are designed for Microsoft Windows Server operating systems. They typically come as a single software package handling all aspects of email, making them easier to set up but potentially less flexible.
Windows mail servers are generally simpler to manage, especially for those familiar with the Windows Server environment. The single software package approach simplifies setup but might limit advanced configuration options.
Microsoft provides security updates and patches for Exchange Server. However, proper administration remains crucial.
Cloud-Based Mail Servers
These servers operate on cloud platforms like Google Workspace or Amazon’s Simple Email Service (SES). The underlying infrastructure and software are managed by the cloud provider.
Management is often the easiest as the cloud provider handles most aspects, such as server maintenance, software updates, and security. Users typically manage their mailboxes and basic configurations through a web interface.
Cloud providers typically handle server security and infrastructure maintenance.
While servers may differ for different email providers, protocols remain consistent as they are involved in transmitting emails across networks. The next section explains the three types of email protocols.
Mail Protocols Explained: SMTP, IMAP, and POP3
SMTP, IMAP, and POP3 email protocols play different roles in the world of email, though they all work together to get your emails where they need to go. Since they handle different tasks, it’s important to understand the differences between them and how email servers use them.
SMTP (Simple Mail Transfer Protocol)
SMTP is used for sending emails from a client to a server or between servers; it handles the process of routing the emails. SMTP handles the process of sending, routing, and delivering outgoing mail.
SMTP typically uses port 25 but also uses ports 587 (StartTLS encryption) and 465 for secure transmission.
IMAP (Internet Message Access Protocol)
IMAP is used for retrieving emails from a mail server. It allows multiple clients to manage and synchronize email accounts. Emails are stored on the server and can be accessed from multiple devices. IMAP is an inbound mail protocol.
This protocol works over port 143 (non-encrypted) or port 993 (SSL/TLS encrypted).
POP3 (Post Office Protocol 3)
Like IMAP, POP3 is also used for retrieving emails from a mail server. A key difference is that it downloads emails to a single device and typically deletes them from the server after download. POP3 is an inbound mail protocol. POP3 and IMAP have different use cases as you will see below.
It uses port 110 for non-encrypted transmission or port 995 when sending data with SSL/TLS encryption.
What is the Difference Between SMTP, IMAP, and POP3?
This table explains the differentiating factors between these three protocols:
| Feature | SMTP | IMAP | POP3 |
|---|---|---|---|
| Full form | Simple Mail Transfer Protocol | Internet Message Access Protocol | Post Office Protocol 3 |
| What is it used for? | Sending emails | Retrieving and managing emails | Retrieving emails |
| Direction | Outbound | Inbound | Inbound |
| Port numbers used | 465, 587, and 25 | 993 and 143 | 995 and 110 |
| Email storage | Does not store emails | Emails stored on the server | Emails are downloaded + deleted from the server |
| Synchronization | No synchronization | Synchronizes across multiple devices | No synchronization |
| Access | Used by email servers to send mail | Allows multiple devices to access and manage mail | Usually accessed by a single device |
| Used by | Mail servers, email clients (for sending) | Email clients such as Outlook | Email clients like Outlook or Thunderbird |
Now that you know about the hardware, software, and protocols — here’s how it all comes together.
How Emails Travel Between Servers
When you click “Send” on a new email, here is the technical process that unfolds behind the scenes.
1. Creating the Email
You craft your message in your email client (webmail, desktop client, or mobile app). This email follows the RFC 5322 format — so it must include the recipient’s email address, subject line, and message body. The email client converts everything into a standard format called MIME (Multipurpose Internet Mail Extensions).
While RFC 5322 dictates the email format (such as email headers and body), MIME allows you to add different media (email attachments such as images or video).
2. Connecting to the Outgoing Server (SMTP)
When you click send, your email client connects to your email provider’s outgoing mail server. This server uses SMTP to handle email delivery.
3. SMTP Server in Action
The SMTP server first verifies the sender’s identity through security protocols, such as Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM). Then, it looks up the recipient’s email address using the Domain Name System (DNS). DNS translates email addresses into numerical IP addresses that computers can understand.
4. Finding the Recipient’s Server
The SMTP server uses the DNS information to identify the recipient’s incoming mail server. This server is also called the Mail Transfer Agent (MTA). The email is then sent to that server.
5. Delivery and Security Checks
The recipient’s MTA receives the email and performs various checks. The spam filter scans for malicious content and viruses to protect the recipient’s inbox. It might also check if the recipient’s mailbox is full or if the sender is blacklisted for spam.
6. POP3 or IMAP Come Into the Picture
If all checks pass, the MTA delivers the email to the recipient’s mailbox. POP3 or IMAP is used to manage how emails are stored and accessed by the recipient’s email client. These protocols determine when the recipient sees the new email notification.
The entire process happens very quickly — within minutes, the email reaches the recipient’s inbox.
What Should Email Marketers Know About Mail Servers?
Email servers are not really something you need to worry about as an email marketer. Unless you plan to set up a private email server to send email campaigns, you don’t have to get into technical details.
Your goal is to work with a reputed email service platform that has its mail servers figured out.
When you link your email host to an ESP, you send your email campaigns through its servers. The servers must enjoy a good reputation with mail providers, or your emails will make a beeline for the spam folder instead of the inbox — also known as poor email deliverability.
If superior email deliverability is what you seek, do your research before you sign up with an ESP. And we have a rock-solid recommendation for you.
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We want our email marketing clients to focus on critical matters such as campaign management and achieving better engagement metrics and leave the mail server management to us. We have robust email infrastructure and usage policies in place to ensure our delivery rates and deliverability rates always perform at elite levels.
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