Sending an email to someone is something that you can do within few seconds. This is one of the main reasons behind the popularity of emails as a communication medium. However, there are numerous underlying technologies, which are enabling this simple functionality to you. SMTP server holds a prominent place out of them.

What exactly is an SMTP server?

The SMTP server will be the server, which is responsible for sending out emails. SMTP stands for Simple Mail Transfer Protocol. It is also capable of receiving emails coming from the email client. The SMTP server of your email client would pass over the email you send to the SMTP server of the recipient's email client. That's how an email you send will be delivered.

Understanding the differences between an SMTP server and an IMAP/POP3

There are many different types of email servers used by people out there. SMTP is the protocol that is being used to send emails. However, IMAP4 and POP3 are the protocols that are being used to receive emails. Both these protocols should work together to ensure the proper functioning of an email system.

How does an SMTP server work?

Here are the basic steps associated with the functionality of an SMTP server.

  • The email client, or mail user agent is connected to the SMTP server associated with your domain. This domain will be the SMTP handshake. The connection between them is established via the SMTP port, which is 25. However, you can configure this port to another one as well. Once this connection is established, the SMTP session would start.

  • You can enter a recipient for an email and send it out. Then the email client of you would submit the email addresses of both sender and recipient, along with the body content and all attachments to the SMTP server.

  • The SMTP server would act in the form of a mail transfer agent. This is where it would check whether the domain of the recipient is the same as the sender. If so, the email will be sent to the POP3 or IMAP server of the recipient directly. If not, the SMTP server will need to communicate that to the DNS (Domain Name Server).

  • The DNS will then provide the IP address of the recipient.

  • The SMT server of the sender would then connect to the SMTP server of the recipient. This is where the email content will be transferred. In fact, the email will be placed within the SMTP queue. There will be a small buffer, which would queue and store the emails before they are delivered to the endpoint recipient.

  • The SMTP server of the recipient would also verify the incoming mail. Once the user name and domain are recognized, the server would then forward that to the IMAP or POP3 server. This is where the email will be successfully delivered.

What are the differences between an MTA and SMTP server?

Most people tend to use the term MTA (Mail Transfer Agent) to refer to the functionality of SMTP servers. However, you should understand that MTA and SMTP servers are two different things. MTA is the name given to the software, which you can find within the SMTP server. The MTA would usually be able to receive the email from MUA, and it will forward the mail to the MDA (Mail Delivery Agent) or a different SMTP server. There are certain situations where you can find the presence of an MSA (Mail Submission Agent) in between the MTA and MUA as well. You should understand this difference and figure out the specific role played by the SMTP server.