A Comprehensive Guide to the Digital Universe of SMTP and IMAP Protocols

Vladislav Podolyako
Jan 12, 2022
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Sending email messages is one of the most comfortable and popular ways of business communication worldwide. What's more, an occasional outgoing email is no stranger to almost every individual these days. We send and receive emails quite often, but we rarely think about what comes after hitting the 'send email message' button. Yet, the process is no less fascinating than it seems to be. Besides, understanding how the whole operation works can grant you valuable insight into the matters of email deliverability, which are crucial for such spheres as marketing and sales.

So, when you center your attention on what goes on behind the scenes, you need to know that various protocols are put to use to make the whole thing work, and the SMTP protocol stands right in the middle of it. There is also a more standard protocol – IMAP. SMTP and IMAP are related but not interchangeable, and it is time you learn why. Folderly experts compiled a detailed and easy-to-grasp guide on the matter for you to understand the core of the subject.

What is SMTP – general introduction of the protocol

Simple mail transfer protocol is how SMTP is deciphered. While the notion still seems confusing enough, it's safe to claim that the SMTP protocol is responsible for sending emails from different email clients. Every time you are about to send an email via any major email client such as Gmail or Microsoft Outlook, the SMTP server is activated. The main aim of the server is to transfer your email message to the recipient's inbox.

Simple mail transfer protocol is no more than a compilation of rules that every email client is familiar with. The protocol enables different accounts to exchange information quickly and efficiently.

What does the SMTP server stand for?

What is SMTP? It is time to proceed with the definition of the SMTP server. The simplest way to define an email server is to describe it as a digital post office. It gets the mail, and it sends it out to the defined recipients. However, before the letter from one city reaches the receiver in another, it goes to a local post office which checks it and approves for delivery. The situation with emailing is closely related to the routine. Yet, it does not take days to receive your message. It often takes seconds.

An SMTP server is a type of app that's been designed to unite other applications under the same roof called an email client. The primary goal of an SMTP server is to deal with sending, relaying, and receiving messages. You shouldn't confuse the server with an SMTP port. The latter is more of an endpoint of communication that tracks the email message as it passes through the network.

How SMTP works: basic principles

It is not enough to know that SMTP stands for simple mail transfer protocol. You should also know how it works. While the general notion is easy to understand, the actual process gets a tad technical, and it may take a few to realize how the operation goes entirely.

So, the moment you establish an SMTP server, every email client known can connect with it and communicate via the server. Once the sender initiates the sending process by clicking the corresponding button, the client will open the connection to the server.

After the opening, the SMTP will instruct the server as to what should be done with the indicated message, where it should go to and what content should be transmitted along with it. The Mail Transfer Agent is a tool designed to check if the recipient's email client coincides with that of the sender. Then, the following routines take place:

  • If the email domains coincide, the email messages are sent immediately.
  • In case the domains differ, the server will use the DNS records to figure out if the mentioned domain is reliable, and after that, the message will be transferred.

In theory, the whole process seems to be lengthy and complicated. However, in reality, it takes no more than a few minutes. Besides, there are services like Mailgun SMTP that can take total care of the sending process without personal interference required.

Primary SMTP client commands

SMTP commands are peculiar codes that power message transition between email servers. There are some basic commands that you can distinguish between:

HELLO – as simple as the command seems to be, it is very powerful since it is designed to trigger the whole process and direct it the right way. This is the step where the email client is being identified, and the server sends the mentioned command spiced with the domain name and IP it represents.

MAIL FROM – after a successful identification, the sender is supposed to share the code, which indicates where the email messages are from. The command tells the server about a new operation that will take place soon so that the server gets ready to accept and process the address. The server will reply with a 250 OK command if everything goes well.

RCPT TO – once the 250 OK code is received, the following command will represent whom it is intended for – the recipient’s address. The server's response will be the same, and you can proceed with providing other recipients’ addresses if there is more than one used to get your messages.

DATA – when you send emails, you pack them with essential content that you want to be retrieved by the recipient. Thus, the mentioned code identifies the data transmission between the email client and the server. Once SMTP servers receive the data, code 345 is sent as the reply. This means that the server holds all the necessary data and is ready to pass it onto the receiver.

QUIT – after the email messages are sent, the client will initiate the QUIT code, which stands for closing the operation. Once it is successful, the server will send the 221 code.

RSET – there are cases when the email transfer needs to be canceled. That is where the Reset command comes into play. It is designed to reset the process without aborting the connection. The code often comes up when there is an error spotted. Usually, this concerns incorrect recipient addresses. 

Common SMTP error codes

No matter how simple and well-designed the sending process may seem, there are common issues with retrieving email messages, and some error codes will inevitably come up. In case anything goes not the way it has been supposed to, the email server will notify you by bringing the error code up. The better you are aware of what these codes stand for, the easier it will be to manage the existing problem.  

Commonly, there are two groups of SMTP error codes to distinguish between:

  • 4.X.X Persistent Transient Failure – A complicated error code means that the email server has temporarily failed. You can repeat the sending procedure and have the error solved without any effort put into the process. Yet, some servers use the action to prevent suspicious emails from getting into the user's inbox.
  • 5.X.X Permanent Error – this type of error code will signify that the SMTP connection dropped. Repeating the procedure will end in the same code, so you should better check the SMTP settings.

What is IMAP or the general introduction of IMAP protocol?

The two abbreviations – IMAP and SMTP – are often seen together, so most people think they are one and the same. Yet, while SMTP is all about the sending process, the Internet message access protocol is designed to manage and retrieve emails from the receiving server.

How does IMAP differ from SMTP?

What is IMAP? It is safe to mention that SMTP servers are centered on sending messages and are often referred to as a type of outgoing message transfer agent. At the same time, IMAP stores the right to handle incoming messages adequately.

Should you own SMTP or imply a third-party service?

It has already been mentioned that there are third-party SMTP services such as SMTP SendGrid. This brings us to the question of whether the own SMTP is better than a third-party one. The fact is that there are benefits and downsides that either option features.

Pros and cons of running the SMTP server on your own


  • As a sender, you won't be limited in the volume of emails to send out.

  • Email deliverability reports will be at hand since every email sent is monitored, and delivery information will be provided.

  • No need to share your mailing list with any third-party partner.


  • The approach is resource-consuming. It will take a lot of time, effort, and means to maintain a private SMTP server.
  • Own SMTP server is local. Thus, power outs and Internet connection drops will interfere with the sending process.
  • It will take time to get deliverability rates to the desired level, and security breaches may occur quite often.

Pros and cons of using a third-party SMTP service


  • Hassle-free management since all is taken care of professionally with as little sender interaction as possible.

  • Security levels are up. Most SMTP services, such as SMTP SendGrid, feature multiple backup options that keep your business safe.

  • A cost-effective approach ensures that while you pay for the service, you free up other resources such as your time and efforts to be directed on something not less important.

  • Reliable third-party services have a good reputation with most ISPs, so your deliverability rates will be a lot higher.


  • The sole drawback of going this way is that you will depend on someone else. Taking the advantages, it is a minor price to pay.

Final words

Now, when you know how both SMTP and IMAP work in detail, you can fully understand the importance of those protocols when it comes to email marketing. One of the easiest ways of checking if these protocols are intact would be to run a quick email delivery test. Folderly is among the most effective services to refer to in times of need. Our expert will offer professional and effective assistance within the shortest notice!

Vladislav Podolyako
Vladislav Podolyako
Founder & CEO
Vlad’s decades of entrepreneurial wisdom and business building experience have allowed him to successfully mentor a diverse group of business owners, entrepreneurs in growing their companies. A recognized expert in the areas of transforming organizational culture and leadership development, B2B Sales, Marketing, spent more than 10 years building technology products, with a background in communication networks and electronic device engineering.

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