How to deliver emails and avoid blacklists: features of an SMTP server

The SMTP server is good for sending emails if you want to increase the percentage of emails delivered and not get spammed. However, it is necessary to know the features of this protocol and choose the type of SMTP server correctly.

What is a SMTP?

Simple Mail Transfer Protocol, or SMTP, is a simple communication protocol for sending emails. What makes this protocol special? It is only responsible for sending emails, not for processing them. You will receive a notification when a message has or hasn't been delivered.

The history of SMTP began in the 1970s when the Mail Box Protocol and SNDMSG were implemented. They were followed by FTP Mail and Mail Protocol, used for email. But only in 1982 SMTP finally replaced FTP in email transmission.

In 2008, the latest update of this network protocol was released, which included a scalable extension - ESMTP (Extended SMTP).

Types of SMTP 

An SMTP server is a server that operates on this protocol and is designed for relaying (transferring) between the sender's server and the recipient's server. This server can be used for personal mail with a limited number of emails sent per day, as well as for mass mailings and transactional emails.

There are some types of SMTP:

• The servers of Internet service providers limit the number of emails sent and the speed of their delivery depending on the pricing plan. However, you do not need to configure the SMTP server - it will be done for you;
• Gmail, Yahoo, and other free mail services allow you to use SMTP. After setting up (you need to log in and specify port and encryption type), you can use mass mailing, but with a limit of 150-500 emails per day;
• Transactional email services can be used to set up sending emails from a website, CRM system or web application. To do this, you need to register an account and specify the email service as the sender in the desired system. These services also have pricing plans that determine limits for sending emails;
• The SMTP server can also be provided by a hosting provider. The most reliable option, which will provide full control, protection from blocking, and the ability to control mailing by yourself, is to create your own SMTP server on a virtual server. Together with VPS, you will get a dedicated IP and the opportunity to use all the advantages of SMTP.

When choosing a type of SMTP server for your project, pay attention to the number of emails. Free mail services will be a great option for a small number of emails, but for larger volumes we recommend a VPS.

How does SMTP work?

SMTP works with almost all transport layer protocols such as TCP, UDP, etc. SMTP is also assigned to ports 25, 587, and 465:

• Messages are transmitted between mail servers via port 25;
• Messages are transmitted from the mail client to the server via port 587;
• Port 465 is used when a secure SSL connection is required.

The SMTP protocol works by using serial commands:

• MAIL FROM - sender's address (address for returned emails);
• RCPT TO - recipient's address;
• DATA - the content of the message (header and body).

The connection is established with the HELO command and terminated with the QUIT command immediately after the message has been sent.

What service is provided by SMTP and how  SMTP works step by step?

The recipient's server first receives information on relevant requests to find the recipient's server. It uses email to identify the recipient's mail provider and asks for the IP address of its SMTP server.

The sender's server makes a connection via port 25 (587/465) and sends the email to the destination server.

If the recipient's server does not respond, the sender's server tries several more times to establish a connection. If it fails, the sender's server will receive a sending error.

If the delivery was successful, the sender's server will be notified accordingly. The email will be stored on the server until it is "opened" using the IMAP and POP protocols.

Each SMTP response has its own code, which comes with a text explanation. The server response can be positive and start with 2 (the form 2**). Negative responses may have a permanent code (5**) and a temporary code (4**). For example, if the code starts with 3 (3**), then additional data is expected. If the code starts with 5, the command isn’t complete at all.

SMTP features: security and spam

Initially, SMTP had no means of authenticating senders, but in 1999 a similar extension of this protocol was introduced. So, with the help of the SASL (Simple Authentication and Security Layer) and user authentication, email transmission became safe. Also, Microsoft uses its own protocol SPA (Secure Password Authentication) with the SMTP-AUTH extension.

We mentioned earlier that port 465 is used to establish a more secure connection. Thanks to the security certificate (SSL), you can guarantee the protection of all data.

However, current security measures still cannot fully rid recipients of spam. That's why most of the time, emails end up in spam at the processing stage rather than at the transmission stage.

SMTP in marketing

For marketing activity, SMTP is a tool with many benefits:

• Low cost. This is due to the main function of the protocol - email delivery. If you choose specialized services, along with templates, constructors and mailing analytics, you get a higher price;
• High deliverability. The SMTP server is accessed only after authorization and, ideally, by using a dedicated IP, so there is less risk of getting caught in spam;
• Security of client data. The SMTP server is only the "postman" and private data is stored only on the sender's server;
• High speed and delivery guarantee. The SMTP server will always notify you if the email was delivered or why it was undeliverable. For example, the sender's address can be added to a blacklist;
• There is no need to install the software. You can learn the general principle of the protocol and the basic commands.

As a tool for sending large numbers of emails, the SMTP server performs very well. In this case, the user receives all the necessary information about the operation of the protocol and does not have to worry about getting caught in spam.