SMTP stands for Simple Mail Transfer Protocol. It is an application layer protocol that allows software to transmit electronic mail over the internet². SMTP is a push protocol and is used to send the mail from one user to another, while other protocols such as POP or IMAP are used to retrieve the mail at the receiver's side¹.

SMTP works by establishing a TCP connection between an email client and a mail server on port 25. The email client, which is the sender of the message, initiates the session and sends commands to the mail server, which responds with reply codes. The commands and reply codes are defined by the SMTP standard and allow the client and server to exchange information such as the sender's and recipient's email addresses, the message content, and any errors that may occur³.

SMTP can be used in two modes: end-to-end and store-and-forward. In end-to-end mode, the email client contacts the destination's host SMTP directly, and sends the message to it. In store-and-forward mode, the email client contacts a local SMTP server, which acts as a relay and forwards the message to another SMTP server closer to the destination, until it reaches the final recipient's SMTP server¹.

SMTP has four components: Mail User Agent (MUA), Mail Submission Agent (MSA), Mail Transfer Agent (MTA), and Mail Delivery Agent (MDA)¹.

• MUA is a computer application that helps users in sending and retrieving mail. It is responsible for creating email messages and passing them to MSA.

• MSA is a program that receives email messages from MUA and passes them to MTA. It also adds additional headers to the messages, such as Date and Message-ID.

• MTA is a program that transfers email messages between SMTP servers. It also maintains a queue of messages and handles retries and errors.

• MDA is a program that delivers email messages from MTA to the recipient's mailbox. It may also perform filtering or sorting of messages based on user preferences.