What is SMTP?

As an email marketing professional, it’s essential to do your due diligence to ensure that your campaigns run smoothly. Part of that process is getting to grips with the underlying mechanisms involved in email communication and the key protocols that make email marketing possible.

The Simple Mail Transfer Protocol, or SMTP, is one such protocol.

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SMTP and its function How does SMTP work? Establishing an SMTP connection
Specifying sender and recipient information
Transferring email data
Closing the connection
SMTP, POP, and IMAP SMTP and ports Port 25
Port 465
Port 587
Port 2525
Conclusion

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SMTP and its function

The Simple Mail Transfer Protocol is a component of the application layer of a suite known as TCP/IP (Transmission Control Protocol/Internet Protocol), a set of rules that determine how data is exchanged on the Internet.

SMTP is the standard communication protocol for sending email messages across networks. The function of the application is to enable mail clients to communicate with SMTP servers, which, in turn, route messages to the appropriate mailboxes.

SMTP is the backbone of all email communication and, as such, it is a fundamental part of email marketing.

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How does SMTP work?

The SMTP process is quite technical, but it can be simplified into a few key stages:

Establishing an SMTP connection

Once a sender has authored an email and pressed “send,” the email client must establish a TCP (Transmission Control Protocol) connection with the outbound SMTP server. From here, the client can engage in a dialog with the SMTP server. This is often referred to as an SMTP conversation and comprises a series of commands and responses from the SMTP client and server, respectively.

At the outset, the client will send the HELO command to the outbound server. This command identifies the SMTP client to the server.

Specifying sender and recipient information

After the SMTP client has introduced itself to the mail server, it will issue the MAIL FROM command. This command identifies the sender of the email and indicates that the client is about to initiate a mail transfer. If accepted, the server will respond with a positive 250 OK code.

From there, the client issues the RCPT TO command. This provides information about the intended recipient of the message. Once again, if accepted, the SMTP server will produce a 250 OK code. This process is then repeated for each email recipient.

Transferring email data

When the server has accepted the sender and recipient details, the SMTP client begins to transfer the content of the email to the sender. This is done using the DATA command and includes information headers, email body, and any attachments.

Once this has been completed, the SMTP server will run the Mail Transfer Agent, or MTA, an application that forwards outgoing messages so that they can be delivered. 

The MTA will use the email address provided using the RCPT TO code to query the Domain Name System (DNS) and obtain the recipient’s IP address. The message will then be sent to the recipient via their inbound SMTP server.

Closing the connection

Once the email message has been sent successfully, the client will inform the server by issuing the QUIT command, signaling its intention to close the connection. In turn, the SMTP server will produce a 221 OK code, which means that the connection has been severed.

Once the connection between the client and the outbound SMTP server has been closed, the server will connect with the inbound SMTP server, which in turn communicates with the recipient’s mail client. Finally, the message is delivered to the recipient’s mailbox.

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SMTP, POP, and IMAP

SMTP is not the only protocol used in email exchange. The other two main protocols are POP, which stands for Post Office Protocol, and IMAP, which stands for Internet Message Access Protocol.

However, POP and IMAP have different functions from SMTP. Though SMTP is used to send mail from client to server, POP and IMAP are responsible for receiving incoming messages.

Despite having the same general function, POP and IMAP differ primarily in terms of storage. When using POP, inbound mail is downloaded to a single machine and subsequently deleted from the server. On the other hand, IMAP allows emails to be stored on the server, enabling users to access them from several different devices.

SMTP and ports

Ports are digital points through which data can be received and handled, and the ports in use can vary from one server to the next. However, the most well-known and commonly-used ports are the following:

Port 25

The original default SMTP port. It was once used for submitting emails from client to server, though it is now primarily used for server-to-server communication, also known as SMTP relay.

Port 465

Port 465 replaced port 25 as the dedicated submission port for a short time, but it has since been deprecated. Due to new security developments, this port is no longer in widespread use for email submission, although some legacy systems still support it due to its former status.

Port 587

Port 587 is the current standard default SMTP port for email submission due to the implementation of Transport Layer Security (TLS) encryption.

Port 2525

Port 2525 is an alternate port that is occasionally used instead of port 587. Although it does not have official status, it is often used by Email Service Providers (ESPs) for secure submission.

Conclusion

The Simple Mail Transfer Protocol is how the world’s email communications operate. Without SMTP, email marketing would not be possible. Having a comprehensive understanding of SMTP and its role will help you effectively avoid and troubleshoot any issues you may have with your send-outs and ensure that your campaign is successful.

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