how does smtp works

Email is one of the primary services running on the Internet. So, what application, protocol, and services are required for email? The email server stores email messages in a database. Email uses the store-and-forward method for sending and storing messages. Email clients communicate with the servers running mail services to send and receive an email. The client-connected server communicates with other mail servers to transport messages from one domain to another.

When sending an email, the client does not communicate directly with another email client. However, both mail clients rely on the mail server to transport messages. The Email process uses three types of protocols: Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), and Internet Messaging Access Protocol (IMAP). The application layer process sends mail using SMTP, but a client retrieves email using POP or IMAP.

Simple Mail Transfer Protocol (SMTP) Operation

The SMTP message formats required a message body with the message header. The body of the message can hold any amount of text; the message header must have a properly formatted recipient email address and a sender address.

When a client sends an email message, the client SMTP process connects with a server SMTP process on port 25. When the client and server set up a connection, then the client tries to send the email message to the server. After the server receives the email message, it either places the message in a local account in case of the local recipient or forwards the message to another mail server for delivery.

If the destination email server is busy or not online, then the SMTP spool message will be sent later. The server periodically checks for the queue and attempts to send messages again. When the message expiration time is over and still in the queue, the message is returned to the sender as an undeliverable message.

The figure above illustrates the technique of message sending. The client sends an email message to admin@fschub.com. The SMTP / POP server-1 will receive the message. Server-1 will check the recipient’s list of local recipients. If found, the message will be placed on the local account. The message will be forwarded to SMTP / POP server-2 if not found.

Post Office Protocol (POP) Operation

The POP server passively listens on TCP port 110 for client connection requests. When a client needs to use the POP service, it requests to start a TCP connection with the server. On establishing a connection, the POP server welcomes the client.

When the client and POP server set up a connection, both exchange commands and responses until the connection terminates. With POP, when clients download email messages, the server removes these messages.

The POP server has a temporary holding area for mail until it is downloaded to the clients. Because there is no central place for email message storage, it is not an attractive choice for a small business that needs centralized storage for backup.

Internet Messaging Access Protocol (IMAP) Operation

The Internet Message Access Protocol (commonly known as IMAP) is another protocol that describes a technique for retrieving email messages from a remote mail server. An IMAP server usually listens on port 143, and IMAP over SSL is assigned port number 993. Unlike POP, when the user connects to an IMAP server, mail copies are downloaded to the client application.

The original messages are reserved on the server until the user explicitly deletes them. Users view copies of the messages in their email client software.

The server stores Incoming email messages in the recipient’s email box. The user retrieves the messages with an email client that uses one of several email retrieval protocols. Most clients support the standard protocols, such as SMTP for sending an e-mail message and POP and IMAP for retrieving email.

The IMAP client can make a file hierarchy on the server to organize and store emails. When a user wants to delete a message, the server synchronizes that command and deletes the message from the mail server.

Differences Between POP, SMTP, and IMAP

The following table summarizes the key differences between POP, SMTP, and IMAP regarding their functions, port numbers, security, email storage, syncing capabilities, offline access, message management, server load, everyday use cases, and examples of applications or services that utilize each protocol.

Feature	POP (Post Office Protocol)	SMTP (Simple Mail Transfer Protocol)	IMAP (Internet Message Access Protocol)
Purpose	Retrieve emails from a server	Send emails to a server for delivery	Access emails stored on a server
Port Number	110	25	143 (without SSL/TLS), 993 (with SSL/TLS)
Security	Typically lacks encryption	Can use encryption (SMTPS)	Supports encryption (IMAPS)
Email Storage	Downloads emails to a local device	Does not store emails	Leaves emails on the server
Syncing	Generally does not sync emails across devices	Does not sync emails	Synchronizes emails across multiple devices
Offline Access	Provides limited offline access to downloaded emails	Does not provide offline access	Provides full offline access to emails
Message Management	Limited capabilities for organizing messages	Primarily focuses on sending messages	Offers extensive message management features
Server Load	Relatively low as emails are typically removed from the server after retrieval	Moderate, as it involves transferring emails to different servers	Relatively high, as emails are stored and managed on the server
Common Use Cases	Used when internet connectivity is limited or sporadic	Essential for sending emails from email clients	Preferred for accessing emails from multiple devices
Examples	Microsoft Outlook, Apple Mail	Sendmail, Postfix	Gmail, Outlook.com, Mozilla Thunderbird