What is SMTP (Simple Mail Transfer Protocol)?

SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used for sending and receiving emails. It is commonly employed by email clients such as Gmail, Outlook, Apple Mail, and Yahoo Mail. Although SMTP can send and receive emails, it is typically used alongside programs like Post Office Protocol 3 (POP3) or Internet Message Access Protocol (IMAP) to handle email storage and retrieval, as SMTP alone has limited ability to queue messages at the receiving end.

What is an SMTP Server?

An SMTP server is an application or computer responsible for sending, receiving, and relaying emails. These servers generally use TCP on port 25 or 587, which identifies specific processes when an internet or network message is forwarded to a server. Network-connected devices come with standardized ports that have assigned numbers for certain protocols and their associated functions.

SMTP servers are always in listening mode. When a server detects a TCP connection from a client, it initiates the SMTP process to send the email. Outgoing SMTP servers send messages for users, and email clients must have the IP address of the SMTP server to read and send emails. To manage issues like spam, server administrators often control which clients can use the server, typically by requiring client authentication or restricting users by their IP addresses.

How SMTP Works

SMTP operates using a client-server model:

1. Sending Email: An email server uses SMTP to send a message from an email client to another email server.

2. Relaying Email: The email server uses SMTP as a relay service to forward the email to the receiving email server.

3. Receiving Email: The receiving email server uses an email client to download incoming mail via IMAP or POP3 and places it in the recipient's inbox.

When a user clicks the "send" button, a TCP connection is established with an SMTP server. The SMTP server receives this TCP connection from a client and starts a connection through port 25 to send the email. The SMTP client then provides the server with the sender's and recipient's email addresses and the email's content. A mail transfer agent (MTA) checks if both email addresses belong to the same domain. If they do, the email is sent directly. If not, the server uses the domain name system (DNS) to identify the recipient's domain and forwards the email to the appropriate server.

Finally, the recipient's email client uses IMAP or POP3 protocols to download and access the received email.

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