What happens when you send an email?

Sending an email involves several steps. The email program first prepares the message, then checks, transmits and finally delivers it to the recipient’s inbox. All of this happens within a few seconds.

How does sending an email work?

Every email begins with a webmail service in your browser or with an email program (also known as an email client) installed on your computer. The technical term for an email program is Mail User Agent (MUA), sometimes also called a Message User Agent.

Once you’ve written your message and clicked send, the process of sending the email begins. But before the message reaches its des­ti­na­tion, it goes through several stages — for­mat­ting, ver­i­fi­ca­tion and for­ward­ing.

The standard protocol for sending messages is the Simple Mail Transfer Protocol (SMTP). Incoming emails are retrieved using either IMAP or POP3.

An overview of how sending an email works

Step 1: The email program prepares the message

When you click Send, your email program prepares the message for sending by dividing it into two parts — the header and body.

The email header contains in­for­ma­tion such as the sender, recipient(s), subject line and the date and time of sending.

Below the header is the body of the email. The body includes the main content of the email — your message text and any attached files such as images or documents. Together, these make up the complete email that will be sent.

Step 2: The mail server checks the message

Next, your Mail User Agent (MUA) connects to your email provider’s mail server. A mail server runs software designed to con­tin­u­ous­ly manage the sending and receiving of messages. This software is known as a Mail Transfer Agent (MTA) (or Message Transfer Agent) and forms the basis of a mail server.

Some providers also use a Mail Sub­mis­sion Agent (MSA) in front of the MTA. The MSA com­mu­ni­cates directly with your email program, checking whether the recipient address is valid and linked to a real domain. Invalid or in­cor­rect­ly formatted addresses are rejected, while valid ones are forwarded to the MTA. In many systems, the MSA function is already in­te­grat­ed into the MTA software.

Before the message is sent, the MTA checks whether it meets the provider’s size limits. Most email services allow at­tach­ments between 4 MB and 20 MB. If the message or at­tach­ments exceed this limit, you’ll receive an error message and the email won’t be sent. The MTA also scans outgoing messages for spam and malware such as viruses or Trojans. If every­thing is in order, the message is queued for sending.

Step 3: The message reaches the recipient’s mail server

In the next step, the MTA de­ter­mines where to deliver the message by looking up the recipient’s mail server in the Domain Name System — unless the MSA has already done this. Once the server is found, the MTA checks whether the local part of the address (the portion before the @ sign) exists. If not (owing to a typo or simply because it doesn’t exist), the MTA returns the message to the sender along with an error no­ti­fi­ca­tion. If the address is valid, the MTA forwards the message to the recipient’s mail server.

During trans­mis­sion, the message — like most data sent over the internet — is divided into smaller data packets. Each packet is sent as part of a TCP segment, typically around 1,500 bytes in size, which is known as the Maximum Trans­mis­sion Unit (MTU). This approach improves ef­fi­cien­cy, as smaller packets move through the internet more smoothly and can take different routes to the des­ti­na­tion depending on which ones are less congested.

These packets travel across the internet through a network of exchange points known as internet nodes. At these nodes, email providers exchange large volumes of data with one another, making them a key part of the internet’s global in­fra­struc­ture. In­di­vid­ual packets of an email pass through these nodes on their way to the recipient’s mail server. Once all packets arrive, they are re­assem­bled to restore the email to its complete form.

Step 4: The email is delivered to the recipient

When the message reaches the recipient’s mail server, the MTA runs another series of checks, this time for:

• Maximum message size (as set by the provider)
• Potential spam (based on sender rep­u­ta­tion and sus­pi­cious keywords)
• Virus and malware

Many providers now also use AI-based filters to detect sus­pi­cious or fraud­u­lent messages more ac­cu­rate­ly.

If the email passes all checks, a Message Delivery Agent (MDA) stores it in the recipient’s inbox. When the recipient opens their email client, their MUA retrieves the message from the server.

The entire process — from sending to delivery — usually takes just a few seconds.