What Is SMTP?

The technology that powers your emails

Illustration showing electric blue lines routing into an envelope, symbolizing email

alengo/Getty Images

SMTP, or Simple Mail Transfer Protocol, is one of the interconnected online protocols and standards that support the modern internet. Just like HTTPS enables secure browsing, and TCP enables packet-based communications, SMTP enables email over the internet.

Thanks to an interconnected network of email servers that follow the same rules, emails can find their way across the wider internet, reliably arriving in their recipient's inboxes only moment after it departs the sender.

What is SMTP?

SMTP is a core element of online infrastructure, as integrated into modern communications as SMS and cell towers. Only through universal application of the same standard can we be sure that many servers that make up the internet can interact as designed.

SMTP is the system we use for sending emails effectively. Email servers have specific SMTP servers, typically addressed like smtp.mailserver.com, which manage sending email for many addresses simultaneously. The format of an email address is part of SMTP's underlying ruleset. The recipient's SMTP server is found based on the email's domain name, and appears after the '@' symbol. The mailbox on that server is found by the name before the '@' symbol.

SMTP is also responsible for associating the return address with every email, but it's a trust-based protocol. If the sender spoofs their email address, SMTP has no way to stop or detect it.

Mapping an Email's Journey With SMTP

SMTP is responsible for moving your email from you outbox to the recipient's inbox with a system called "store and forward." Messages are sent to intermediate servers, called nodes, then organized and forwarded on as needed. This produces a chain of locations where a message might be stored temporarily, as it hops from node to node.

Thanks to these intermediate nodes, the SMTP network is delay tolerant, meaning it can manage intermittent connections to the service. It also doesn’t require a direct connection between the sender and the recipient. Most postal services use a similar system, sending mail to intermediate hubs for the most effective sorting and distribution.

Drawing showing the SMTP transfer model. Blue arrows can be implemented by SMTP variations.
Ale2006-from-en [ CC BY-SA 3.0]

The user writes an email in their preferred email client, called a Mail User Agent (MUA). It's submitted to a Mail Submission Agent (MSA), which can report or correct minor errors in the message.

The MSA sends the message on to a Mail Transfer Agent (MTA), which uses the DNS system to look up the address of the Mail Exchange (MX) server associated with the domain name of the recipient's email address. The MX server then uses its internal records to pipe the message to Mail Delivery Agent (MDA), which finally hands the message off to the recipient's inbox.

If email is being sent within the same domain, such as between employees at the same company, the message may not need the MX server. Not every email will follow all of these steps, but most messages are sent with chain of communications like the one above.

Why Does SMTP Protocol Matter?

Why are compatible standards so important? For a comparison to the analog world, think of the format for writing addresses on a letter. Every country has its own system, and the formats don't easily interoperate. If you're not familiar with the format of an address, you need to first convert the address into an understandable format.

Anyone who's actually worked with international mail can tell you how complicated it gets, which can lead to errors, delays, and lost messages. Because SMTP is a universal standard, no conversion or translation is required, ensuring messages are correctly delivered without delay or damage.