SMTP - Simple Mail Transfer Protocol

SMTP stands for Simple Mail Transfer Protocol. It is a worldwide email transmission standard for receiving email. Mostly, the IMAP (Internet message access protocol) is used. Earlier In the 1960s, one-to-one email communication was used. People used specific mainframe systems in this. As more computers were introduced in the network while the internet was developing, some standards were developed to allow users of the different systems to communicate through email. SMTP was found and grew in the 1970s from one of those standards.

Features of SMTP
In addition to simply just transferring mail from server to server, SMTP also has some extra features. Some of them are described below.

Mail Relaying: Once SMTP was wildly used to route email from one server to another server before reaching its server. But now emails are transferred directly from sender server to receiver server using DNS MX records (To verify the receiver’s SMTP address)

Mail Forwarding: This is not just forwarding like any other email forwarding, but it’s the ability to transfer mail from any non-local server to an appropriate destination. If a user uses a company’s mailbox and switches to another company, then the company does not allow the user to login to the server, but they forward any incoming email to the user’s personal mailbox.

Address verification: SMTP uses a “verify” command to verify the existence of the email address before sending an email.

Security: SMTP servers support TLS (Transmission layer security) to encrypt the communication channel.

Functioning of SMTP
SMTP works closely like MTA (Mail Transfer Agent). It supports the text/string-based transmission and opens port number 25 on the SMTP server. To generate an email, SMTP first checks the system’s configuration from which the email is generated. When that satisfies the requirement (installation of the server). SMTP uses a technique called forwarding/pushing for sending emails between SMTP servers. It also ensures the delivery of the email. If the email is not delivered, then it returns to the sender.

To understand this, we can divide it into several categories.

Generating email
Identifying email type
Forwarding
Storing the email

1.Generating email: To write an email, a user needs an email client application like a browser (Google Chrome). The user types the mail server address in the URL bar and opens its mailbox. Finally, the mail is written with text and attachments and stored on the mailbox server.

Identifying email type: When the user clicks the send button, the SMTP then comes into action and identifies the email type, since SMTP alone does not support attachments like pictures and document MIME (Multipurpose Internet Mail Extensions) is used as a support to extend the functionality of email, mailbox server adds the MIME header at the beginning of the transmission. Then the header is identified for any media content.

3.Categorization of email data: After identifying email content, SMTP allows the servers (the network computers which handle incoming and outgoing emails) to break the data into categories so the outgoing server can understand how to send the data.

3.Forwarding: After the categorization is done, the SMTP pushes the email data from server to server leaving a temporary copy. When the email is reached to its destination server, it informs the sender of its delivery. Also, the information of each server address is stored in the email header, so spam can also be identified.

4.Storing the email: After the email reaches its destination server address, the email is stored there in the mailbox. The email stays there as long as the user does not delete it. The user can fetch the email using the email client applications. The text/media content is then identified and shown.

When a user wants to send an email (sender) to another user (receiver), Sender generates an email on an SMTP server like Gmail using a web browser. This email is then identified for attachments and media type content and forwarded/pushed to another SMTP server like Yahoo. If a user wants to access that email, the pushed email is stored on the receiver server. POP (Post office protocol) or IMAP can be used to fetch/pull the email.

Application of SMTP
All important email providers use SMTP to transfer emails between servers like Gmail, Yahoo and Apple. SMTP is wildly used in every mail server. The printing server also uses SMTP for cloud-based printing.