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SMTP popular science introduction, how does it work?

Simple Mail Transfer Protocol Explained – What is SMTP and How Does It Work? Email communication has become one of the most integral parts of our personal and professional lives.

  Simple Mail Transfer Protocol Explained – What is SMTP and How Does It Work? It’s hard to remember a time without email — but yes, there was. It’s also hard to imagine today without e-mail communication. Email communication has become one of the most integral parts of our personal and professional lives.

So it’s an understatement to say we all owe the Simple Mail Transfer Protocol ( SMTP ). In this article, we’ll take a closer look at this component of modern communication methods.

What is an agreement?

In networking terms, a protocol is a standardized set of rules for formatting and processing data that enables connected devices to communicate with each other.

Although connected devices may use different operating systems, software, and hardware, they can communicate with each other using common protocols.

A standardized protocol acts as a common language between two connected devices.

For example, Windows PCs can communicate with Linux machines because they both use Transmission Control Protocol/Internet Protocol (TCP/IP) to connect to the Internet.

What is SMTP?

SMTP or Simple Mail Transfer Protocol is an Internet standard communication protocol for e-mail transmission. It was invented in 1981.

This protocol enables mail servers and other message transfer agents (MTAs) to send and receive electronic messages. For example, it’s the protocol we use when we send email over the Internet.

Many of the email clients we use every day (including Gmail and Outlook  Yahoo, etc.) use SMTP to “push” or send messages to their respective recipients.

This protocol is sometimes paired with IMAP and POP3 which handle message retrieval , while SMTP primarily sends messages to a server to complete the email sending and receiving cycle or process.

SMTP only sends text

One thing we need to consider is that SMTP can only transmit text messages. In other words, it cannot be used to send any other format.

However, it can be used with another protocol that helps it carry and transmit images as attachments. This “additional” protocol is called Multipurpose Internet Mail Extensions ( MIME ), and encodes all non-text data into text format.

Once the images reach their destination, they are converted back to their original format.

What is an SMTP port?

The SMTP port is the communication endpoint of a connected device that has been assigned the task of transmitting its email data over the Internet and uses SMTP – it is represented by a number.

When an email is sent to another domain, the port number is appended to it, letting it know which port it should go to for proper delivery.

If we send an email to domain with IP address and receive email  using SMTP port 587 , the “send email to” address will be xxx:587  .

What is an SMTP server?

An SMTP server is a dedicated machine that handles sending, receiving, and relaying e-mail over a network— whether on a local area network (LAN) or over the Internet.

When a user clicks the Send button to send an email, the email client sends a connection and communication request to the SMTP server.

The SMTP server accepts the request, opens the connection, and starts accepting instructions for transmitting email. The information contained in the description tells the server the email addresses of the sender and recipient, and the contained message.

After the details are confirmed by the mail transfer agent (MTA), the email is relayed to the recipient’s domain’s SMTP server, which then delivers it to the recipient’s email client.

Type of SMTP server

There are two types of SMTP servers:

  • Generic SMTP Servers – These are generic SMTP servers that relay email between its source and destination.
  • SMTP Proxy Servers – These servers authenticate emails before forwarding them to recipients. They can be configured to be bi-directional and handle inbound and outbound email authentication.

How does SMTP work? – Examples of real use cases

SMTP commands are a set of codes that facilitate the transfer of email messages between servers. Some basic SMTP commands include:

  • Hello, HELO, EHLO  – commands to start the email sending process. An email client identifies itself to an SMTP server using information such as its domain name and IP address.
  • MAIL FROM  – This uses the email address to identify the sender of the email, initiates the transaction, and ensures that the server is ready to accept the email. When ready, the server replies with a 250 OK reply code.
  • RCPT TO, Recipient To  – This command targets the recipient or recipient of the email. The server replies again with a 250 OK reply code. Repeat this cycle for each recipient in the address.
  • Data – Now it’s time to transfer the data from the client to the server. All content is moved to the server. After receiving all the information, the server replies with a 345 reply code indicating the end of the message, followed by a dot to indicate the end of the message. Waiting to accept and prepare to deliver another 250 OK code. Then send the message to the recipient.
  • QUIT  – Once the message has been forwarded, the email client sends a QUIT command to terminate the connection with the server, and the server replies with a 221 code.
  • RESET  – This command is sent to the server when it is time to terminate an email transaction. All variables are reset and the whole process can start over. It is usually used when something goes wrong and the process needs to be repeated from scratch.

This back and forth communication between client and server looks something like this:

How SMTP works

Common SMTP Errors

Just like any other technical process, the email process can lead to errors. Messages can bounce, be blocked, or simply be rejected, in which case the recipient server notifies the client with an SMTP error code.

There are two common groups of SMTP errors:

  • 4.XX Persistent Transient Failure  – Indicates that the mail server has a temporary failure. Although servers often use these codes to block untrusted senders, it can be worked around by repeating the command over and over again.
  • 5.XX Permanent Error – Indicates that the SMTP connection has been dropped and will continue to be made until it comes back online.

Advantages of having an SMTP server

We’ve seen that SMTP is an important protocol that underpins the way we communicate over the Internet. It’s an integral part of the office environment, and it makes sense for businesses to have their own SMTP server.

Some good reasons for a business to have its own SMTP server include:

  • Full Control of Servers – Businesses should have full control over their servers, including their SMTP servers; administration, monitoring, administration, recovery, security and backup of servers are fully under the control of the business.
  • Unlimited Email Sending and Account Creation – Most third-party SMTP relay services have a limit on the amount of email that can be sent. Of course, customers can purchase packages with unlimited sending options, but this comes at a price. You can create as many accounts as you want.
  • Email Tracking – Owning a server means businesses can track emails; disputes over whether emails were sent and delivered can be easily resolved by looking at the records.
  • Prevent being blacklisted – Businesses can protect their reputation and prevent their IP addresses from being blacklisted.
  • Email Verification – Businesses can protect themselves from spammers, emails from malicious users, and emails with malicious content sent from unverified addresses.
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