Simple Mail Transfer Protocol (SMTP) is one of the standard protocols for the transmission of electronic mail across the Internet.
SMTP is used primarily on the mail server side to send and receive emails. Messages are relayed to mail servers from client applications via SMTP, but users typically don’t receive their emails through this protocol. To receive messages, most client applications use client side applications such as the Internet Message Access Protocol or the Post Office Protocol (POP).
SMTP is a connection-oriented protocol, which means that a connection has to made between both points of communication before information can be transmitted. SMTP transmits information over a transmission control protocol (TCP) connection.
When email is transmitted via SMTP the entire sequence is called an SMTP session.These sessions consist of commands which originate from sender’s email client via SMTP and the responses from the SMTP server of the receiver. A session opens, transmits/receives data, and closes.
These stages of a session are called SMTP transactions.There are three transaction types: Mail (discerns the return address), RCPT (establishes the receiver’s address), and Data (message content).
Email Delivery Flow
Emails are transmitted through a series of steps. An email message is first generated by the user and is submitted via a mail user agent (MUA) to a mail submission agent (MSA) using the SMTP protocol.
The MSA distributes received mail to mail transfer agents (MTA) which are responsible for delivering mail to the end user. The MTA finds the correct end user using domain name system (DNS) to look up the mail exchanger record (MX record) for the end user’s email domain.
The MX record takes the message and transmits it to a mail delivery agent (MDA) which transmits the message to the actual recipient email server, using the SMTP protocol. The end user then uses client software to retrieve the email, typically using POP or IMAP protocols to retrieve the email from the MUA.
It is important to understand the SMTP is a delivery protocol only, which is why POP and IMAP are vital to the email process. SMTP cannot retrieve email from a mail server. This is outside of its design capabilities.
Even with all of these steps, the speed of networking systems and end user systems, emails can be delivered in seconds. It is this deliverability speed that makes email such a valuable business asset. Information, including attachments, can be delivered to recipients near instantaneously.
The hijacking of email addresses by spammers, and using unauthorized SMTP servers to deliver multitudes of spam emails has been an issue in the past. Early SMTP servers attempted to combat this issue by restricting access by location, which had limited success, and was impractical from a user standpoint.
Users, particularly mobile users, are rarely sending mail from the same static internet protocol (IP) address, and require a more flexible authentication solution. Modern mail servers are able to restrict access using client authentication to provide access.
Email Server Solutions
While most individual user’s email needs are met by their ISP, or via web based email systems such as Gmail, organizations and business need a different approach. Individualized email servers and domain addresses are an important part of a companies communication strategy.
Third party solutions are cost effective, low stress ways to provide the email uptime and rapid delivery that companies and organizations need in today’s connected world.
This is a guest post. Jonathon Blocker is knowledgeable about SMTP email delivery services and the features that are included. Jon refers to Send Grid online for further information about SMTP delivery.
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