What is SMTP (Simple Mail Transfer Protocol)?

What is SMTP?

The English abbreviation SMTP stands for “simple mail transfer protocol”. It denotes a special communication protocol used to send emails. Despite its appearance back in 1982, the protocol continues to be in demand today. SMTP cannot interact with incoming emails; it is needed exclusively for sending and successfully delivering the email.

The protocol uses its own server for accurate, stable operation. What are other significant features of SMTP? What are its advantages? Why does its 40-year history not prevent it from remaining popular and in active demand both in commercial structures and among private users?

What is it for?

SMTP is primarily used to solve two important problems:

• Checking the correctness of the mail service configuration, issuing permission to send an email. In the process, the protocol must “establish” which device is the recipient.
• Transferring the letter to the specified address, confirming the successful completion of the operation. If sending is impossible, a notification is sent recording the fact of the error and its probable causes.

Popular functions

In addition to basic tasks, the protocol is suitable for solving some secondary ones:

• Organizing unlimited sending of letters. Many providers set limits on the volume of transmitted email. As a rule, they are tied to time periods, weekly or daily. Exceeding the limit leads to blocking. Thanks to SMTP, it is possible to successfully bypass these restrictions without unnecessary troubles and negative consequences.
• Mail cataloging. Our own server provides effective automatic checking of messages. Spam mailings immediately end up in the appropriate directory, safe, verified ones - in the main one, where incoming messages are stored.
• Correct error notifications. Support for many commands makes it possible to understand which messages did not reach the recipients, why the failure occurred.

With high efficiency, this protocol is more accessible than standard email distribution technologies.

Working algorithm

SMTP protocol can be compared to a postman, whose task is to extract a letter from a mailbox and deliver it to the recipient. The protocol “extracts” the letter, studies the specified information, determines the fastest route of transmission. If the initial data is incomplete, inaccurate, then additional input data will be required, if the delivery is not successfully completed, an error notification will be sent.

The rules according to which SMTP functions are as follows:

• Accurate indication of the recipient and sender of the email. This information is the “starting point” for the protocol, successful delivery without it will be impossible.
• Transferring requests to the recipient and sender's server. This operation allows the system to verify the existence of both parties involved in the transfer.
• Sending an email to a specified address.

During operation, the protocol sends certain commands, each of which must be followed by a certain response. The list of the most popular commands is as follows:

• HELLO. A greeting command, which is needed to establish communication. Its execution requires specifying the domain and the email address.
• MAIL. The email address of the party sending the email.
• RCPT. A command to display the recipient's address. There may be several recipients, if necessary.
• DATA. A command that lets the receiving server know that a letter is coming soon. Sending occurs only after such preliminary notification.
• QUIT. The final command. Its submission indicates that the communication session was successful, the email was sent to the right address.

The stages of the SMTP protocol can be described as follows:

• The corresponding command is sent to the server sending the email, after which it begins searching for the recipient server. The provider is determined, the IP address of the destination device is recorded, and the message is sent to it.
• Establishing a connection. Port 25 is used for a stable communication session.
• If there is no response from the receiving party, several attempts are made to resend. If necessary, you can precisely set the required number of such retries. If there is no positive result, an error notification is displayed.

On the issue of ports

As noted above, port 25 is most often used for the SMTP protocol. Some organizations providing network access services, however, block access to it in order to eliminate spam. In such a situation, it is necessary to use alternative solutions:

• 465. One of the most reliable, secure ports, supporting work with SSL certificates and encryption algorithms.
• 587. A port that allows you to cope with outgoing spam emails. To effectively solve this problem, authentication is used on the sending side.

SMTP is a reliable protocol, problems with its use are extremely rare. The main reason for such reliability is the general simplicity of the working algorithm, which allows you to quickly record and eliminate possible errors.

SMTP servers: classification

First of all, SMTP servers can be represented as standard and dedicated, relay. The server class determines the principle of its operation and the result provided. For example, standard servers are needed to exchange personal emails. In most cases, they are used by hosting and regular mailing services. A characteristic feature of such servers is the limit on the number of emails sent. The function of relay servers is to send transactional messages. In this case, there are no restrictions, so you can not be afraid of blocking the IP address.

SMTP servers can also be classified according to the source:

• Provider. The main advantage is the lack of need for configuration and setup, the disadvantages are the limit on the number of emails sent, slow sending. The terms of use of such servers are determined by the subscriber tariff plan.
• Free specialized mailing services. Such a service is available in most major mail services, including Google and Yandex mail. It is available subject to the simplest registration. In the account, you need to indicate the number of the port used and the encryption algorithm. A convenient and effective way to conduct mass mailings, however, free use implies restrictions on the number of letters sent.
• Hosting providers. Such servers are available to website owners. However, they should be used with caution, avoiding mass spam mailings, so as not to encounter IP address blocking. For greater security, it is recommended to rent a VPS server. It opens up wide management options and eliminates the risk of IP blocking.
• Transaction services. They are needed to fine-tune the sending of emails via SMTP from various sources, which can be, for example, websites and applications.

When selecting a source, first of all, you need to take into account the goals pursued, the tasks set. Small organizations can limit themselves to inexpensive, or even free, tariff plans, and send out mailings using standard tools. For large enterprises sending letters to thousands of clients, this option is not suitable; it is recommended to use VPS or special services for transactions.

Advantages of SMTP

The SMTP protocol is used not only by private users, but also by organizations, and is a powerful tool in Internet marketing. Its main strengths in this case are as follows:

• Guaranteed delivery of the letter to the recipient. To combat spam, providers either limit the number of messages sent or completely block this option. In the case of SMTP, this is irrelevant.
• Security of information transfer. The SMTP server does not store the information being sent, its task is only to forward the message.
• Ease of use, no need for additional software. Successful completion of the task is possible even without the relevant experience.
• Detailed reports on successful delivery or errors that prevent it from being completed. Notifications allow you to understand what exactly the problem is and fix the failure.

With all its advantages, SMTP is an inexpensive protocol to use. If you compare the cost of sending letters in this way with specialized services, it will be several times lower.

Summing up

So, SMTP is a simple protocol that allows you to set up sending emails. It assumes a linear principle of operation, is needed specifically for the successful implementation of the delivery process, in case of failure - will send a corresponding notification, a warning with a description of the error. It can be used for both single messages and mass ones. In the absence of strict requirements for additional functions of e-mail exchange, experts recommend using this protocol as the most effective, accessible and easy to learn.

Author of the article:

Editor-in-Chief

Danaev Almaz