An email does not just reach the recipient as soon as you press the send button. An email does not follow a straight path to reach the recipient. It has to complete numerous transitional steps of an ordered process. The main players of this clearly ordered process are the SMTP servers.
What is SMTP?
SMTP stands for Simple Mail Transfer Protocol. It allows the transmission of email messages from an application over the internet. To put it out in simpler words, SMTP handles all outgoing emails. It ensures that the e-mail reaches the intended recipient by the protocol regulations. One important function that the SMTP server performs is preventing spam e-mails from being sent. It uses authentication mechanisms that require an authentic username and password before an e-mail can be sent. This allows only authorized users to deliver their e-mails.
Most of the modern mail servers are made to support the protocol extension ESMTP with SMTP-Auth to enable the authentication mechanisms. To make the movement of e-mails between sender and recipient possible, the internet has a widespread network of distribution stations like these.
How Does SMTP Server Work?
SMTP servers forms an essential link in the process of e-mail transmission, which involves several servers, namely:
- The outgoing mail server of the sender
- External forwarding servers (which may be one or more)
- The incoming mail server of the recipient
Outgoing Mail Server (of the sender)
The program or the webmail application that is used to send an e-mail is called the SMTP client. It is also termed as mail transfer agent (MTA). When the sender hits the send button, the SMTP client converts the message in the e-mail into a header and body and then loads it on the outgoing mail server, which is the SMTP server.
The server MTA or the mail transfer agent is representative of the software basis that is being used to send and receive e-mails. The mail transfer agent analyzes the size of the email and ensures that the e-mail is not a spam. Once this step has been completed, it saves the mail.
With the continuous outflow of emails, the Mail Transfer Agent (MTA) may get overburdened. For this reason, sometimes, the Mail Submission Agent (MSA) checks in advance whether the e-mail is valid or not. After the Mail Submission Agent has validated the e-mail, the Mail Transfer Agent checks the DNS (Domain Name System) for the IP address of the recipient.
External Forwarding Servers
The e-mail will be delivered directly if the domain of the recipient is connected to the mail server that is the same as the sender. However, if the mail servers of both parties are not same, the Mail Transfer Agent breaks the e-mail into small packets of data. These data packets are forwarded to the SMTP server of destination in the shortest possible time, through the route that is least congested.
Incoming Mail Server (of the recipient)
The data packets are reassembled when they reach the SMTP server of the recipient. This completed e-mail is checked for spam once again by the Mail Submission Agent. The e-mail is then transferred to an area of message storage in the incoming mail server. The Mail Delivery Agent (MDA) takes it up from this point to deliver the mail to the inbox of the recipient. Internet protocols like IMAP and POP3 download the e-mail to the SMTP client of the recipient.
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