Simple Mail Transfer Protocol

 Simple Mail Transfer Protocol

“SMTP” is the email delivery company, see SMTP (company). For Short Message Transfer Protocol.

User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 as per RFC 8314. For retrieving messages, IMAP and POP3 are standard, but proprietary servers also often implement proprietary protocols, e.g.The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission. As an Internet standard, SMTP was first defined in 1982 by RFC 821, and updated in 2008 by RFC 5321 to Extended SMTP additions, which is the protocol variety in widespread use today. Mail servers and other message transfer agents use SMTP to send and receive mail messages. Proprietary systems such as Microsoft Exchange and IBM Notes and webmail systems such as, Gmail and Yahoo! Mail may use non-standard protocols internally, but all use SMTP when sending to or receiving email from outside their own systems. SMTP servers commonly use the Transmission Control Protocol on port number 25.

SMTP at work.

SMTP provides a set of codes that simplify the communication of email messages between email servers (the network computer that handles email coming to you and going out). It’s a kind of shorthand that allows a server to break up different parts of a message into categories the other server can understand. When you send a message out, it’s turned into strings of text that are separated by the code words (or numbers) that identify the purpose of each section.

SMTP provides those codes, and email server software is designed to understand what they mean. As each message travels towards its destination, it sometimes passes through a number of computers as well as their individual MTAs. As it does, it’s briefly stored before it moves on to the next computer in the path. Think of it as a letter going through different hands as it winds its way to the right mailbox.

What is an SMTP Server?

An SMTP server is a computer or an app that is responsible for sending emails. It functions following the Simple Mail Transfer Protocol (SMTP). An SMTP server receives emails from the email client. Then it passes them on to another SMTP server and relays them to the incoming mail server.

How an SMTP server works

Take a look at the basic steps of email sending and what role the SMTP server plays.

  • A mail user agent (MUA), which can be your email client or an app, connects to the SMTP server of your domain . This is called an SMTP handshake. The connection is carried out via an SMTP port, which is normally 25. But there are other options you can learn in our blog post about SMTP ports. Once connected, the SMTP session begins.
  • The client submits the sender’s and recipient’s email addresses, as well as the email body and attachments, to the server.
  • The SMTP server, or more precisely the mail transfer agent (MTA), checks whether the domain name of the recipient and the sender is the same. If it is, the email goes directly to the recipient’s POP3 or IMAP server. If the domains are different, the SMTP server has to communicate with the Domain Name Server (DNS).
  • The DNS provides the recipient’s IP address.
  • The sender’s SMTP server connects to the recipient’s SMTP server and relays the email. If the recipient’s server is not available (down or busy), the email will be put into an SMTP queue. This is a buffer where the emails are stored before they hit the endpoint. For more on this, read What is an email queue? Alternatively, the email can be dropped to a backup server.
  • The recipient’s SMTP server verifies the incoming email. If the domain and user name has been recognized, the server forwards the email to the POP3 or IMAP server.
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