SMTP explained: how email really gets delivered behind the scenes

Emails are already a part of job-related tasks, along with some private communication. People use email services on a daily basis, but hardly ever bother about the technology behind this complex process. Generally, the entire process is connected with the SMTP protocol, and we will discuss all the aspects of SMTP in the article.  

Understanding SMTP: the basics

Simple Mail Transfer Protocol or SMTP is used for sending an email from one user to another via the Internet. This is the most commonly used protocol, and there is a diversity of others, such as IMAP and POP. Protocols are sets of rules that help to exchange emails between users.

SMTP is dedicated to email sending via a variety of popular services such as Gmail, Outlook, Yahoo, and others.

What role does an SMTP server play?

SMTP server plays a crucial role in the whole process of receiving, sending, and relaying of messages. It is like an application that gives service to other apps within the network.

The server may be compared with the post offices that receive an email and redirect it to the final destination. However, the entire process happens during a couple of minutes max. Term SMTP port relates to communication endpoints that deal with the transfer of emails.  

How the SMTP protocol functions

After establishing the SMTP server, it is possible to connect and communicate through it. Once the user presses “send”, SMTP connection opens to the server via the email client. After that SMTP client through some commands inform the server about the process of transferring, such as addresses and content. Transfer Agent or MTA checks if both emails are from the same domain, and if yes, the email is sent.

Core SMTP commands explained

SMTP commands are codes necessary for the transmission of messages, and here are some of them:

• HELO or EHLO: basic command for the beginning of the sending process. This means the email client is identified with the SMTP server.
• MAIL FROM: This command is usually done after the initiation to share information about who is sending the mail. Once that is done, the server is ready to accept the email address, and when ready, the 250 code will appear.
• RCPT TO: This command identifies the recipient. The server reacts with the same code, and after that, another recipient can be added. This process can be done as many times as necessary.
• DATA: This command is necessary for the data transfer between the server and the client. Once the message content is saved on the server, the response code will be 345. After saving on the server, it is transported to the recipient, and the same 250 code specifies the sending process.
• QUIT: after the sending process, the break command is sent between the server and the email client. After the successful closer, 221 code appears as a reply.
• RSET: This command is just a reset, and it doesn’t close the connection. All the previous information is removed. This should be done when there was a specific issue and sending wasn’t successful.

These are only the general commands, and there is a diversity of others connected with the security, authentication, and other processes.

Common SMTP error codes and their meanings

Not all emails are smoothly sent to the recipient, there might be a variety of blocks, bounces, and other issues. During such a situation SMTP server uses codes for the notification of the problem.

Let’s divide all the possible issues into 2 major groups:

• 5.X.X Permanent Error: after “5”, there will be 2 other numbers. Usually, such errors show that there is something wrong with the SMTP connection. The resending of an email won’t help in this scenario.
• 4.X.X Persistent Transient Failure: This signifies the temporary issues with the server. That means when the server repeats the command, the issue might be solved.

SMTP vs. POP and IMAP: key differences

Besides SMTP, there are other popular protocols, so let’s review the major differences between the 2 most common receiving protocols. As already clear, SMTP is a sending protocol, whereas POP and IMAP are receiving protocols. Now, let’s dive into the deeper explanation of how everything functions.

What is POP (Post Office Protocol)?

Post Office Protocol is necessary to receive incoming emails, and its current version is POP3. It functions by holding the message for the recipients until they pick it up. All the messages are stored locally, and that might be beneficial for users who want to check their email while being offline.

The difference between SMTP and POP:

• POP is a protocol of acceptance, while SMTP is the sending protocol.
• POP is used only once when between the mail server and the receiver. SMTP is used twice, when sending information from the sender to the server and when connecting to the recipient.   

What is IMAP (Internet Message Access Protocol)?

Internet Message Access Protocol is needed to store email on the mail server. The difference of this protocol is that it uses the cloud server and can be accessed by any device. This protocol is considered to be much more efficient and convenient.

Comparing IMAP and SMTP: what sets them apart?

When comparing with SMTP, it is a sending protocol, and IMAP is responsible for the handling of incoming messages.

Should you host your own SMTP server or use an email provider?

There is no ideal choice of whether you should set up your own SMTP or use a 3rd-party service. Everything depends on your unique needs and lots of individual factors. We decided to share both pluses and minuses of the 2 options, so you can make a more informed decision.

Benefits and drawbacks of hosting your own SMTP server

Pluses:

• No limitations according to the sending volumes. Usually, service providers have daily limits of emails that you can send, but with your own SMTP, you don’t have such boundaries.
• Full monitoring of the delivery process. You will receive detailed information regarding the delivery process. So, it is possible to examine the information and possible errors.
• Privacy. You won’t need to share the email list with anybody, so your customers’ information and your business info are more private.

Minuses:

• Security and delivery problems. It might take some time to get a professional level of delivery. Also, you will need to protect your server on your own from various online threats, and that requires some professionalism in this sphere.
• It’s local, so your server can be very vulnerable to problems with the internet connection or power outages. That means the availability of a backup server is a must.
• Requires more financial investment and time. Except for setting up the server, there should be constant monitoring by professionals.

Pros and cons of third-party email services

Pluses:

• Fewer concerns about technical aspects. You don’t have to control every technical aspect of the server, monitor its performance, or hire IT experts. Everything is done by the email provider, and you can even consult with the professional support team in case of any issues.
• Fewer issues with the deliverability. Because third-party email services have long-lasting relations with lots of providers and ISPs, they usually adjust to the changing requirements and solve issues way quicker. Moreover, they have a huge team of experts that constantly monitors all the processes.
• More secure. Lots of security tools and mechanisms are used to guarantee a secure environment. Also, providers use backup servers to minimize possible risks.
• Cost-effective. You are not investing in the IT professionals who are necessary for the constant monitoring of the processes and lots of other financial aspects.

Minuses:

• Dependency on others. In case of some technical issues, you are entirely relying on the professionals and support of others, and don’t have a huge impact on the process.
• Agreeing on 3rd-party terms. When agreeing on some terms, you will surely have some limitations, so be prepared for this.