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What is SMTP?

Simple Mail Transfer Protocol (SMTP) is a set of guidelines to send emails across the internet. 

This protocol is configured by a mail client – a desktop application that manages emails – to make sure the email messages get delivered to the right recipient.

SMTP uses Transmission Control Protocol (TCP), a connection-oriented protocol which guarantees successful email delivery. 

SMTP models

There are two types of SMTP models:

  • End-to-end – to send messages between different organizations
  • Store-and-forward – used within an organization

Default SMTP ports

These are the standard transmission channels used by SMTP to transmit email messages:

  • Port 25 – used to send messages between mail servers
  • Port 465 – the connection is established through Secure Sockets Layer (SSL)
  • Port 587 – the default mail submission port equipped with Transport Layer Security (TLS) encryption for secure connection

What is an SMTP server?

An SMTP server is a program that runs SMTP. It’s also known as an outgoing email server.

SMTP servers have addresses set by the mail clients or email providers. These are usually formatted as smtp.serveradress.com.

For instance, Yahoo! Mail’s STMP server address is smtp.mail.yahoo.com, while Gmail uses smtp.gmail.com.

Since an outgoing mail server can only send a limited number of emails, many businesses that run an email marketing campaign or send thousands of transactional messages every day choose to use SMTP service providers.

SMTP services are third-party applications that ensure you have a secure SMTP relay for fast and reliable email deliverability. 

How does SMTP work?

An email message needs to be transformed into a set of code before it is sent. Mail clients use SMTP to do the transformation process.

SMTP can only transform plain text into code. Because of this, it works together with Multipurpose Internet Mail Extensions (MIME) to encode non-text content like fonts, graphics, attachments, and so on. 

After encoding the email message, SMTP will transfer the code to an SMTP server. 

It will then verify the code to confirm that the message is legitimate. It will also check the recipient’s email address by looking for its domain located in the Domain Name System (DNS).

For instance, if you’re sending a message to username@ymail.com, the SMTP server will locate ymail.com and transfer the email to that specific destination.

Once the encoded email arrives in the recipient’s mail server, it will be converted into a human-readable message by either POP3 or IMAP protocol ‒ both of which are protocols for receiving emails.

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Olivia is a writer for Zyro and an eCommerce know-it-all. Having spent many years as a retail buyer, she loves writing about trend forecasting, brand building, and teaching others how to optimize online stores for success. She lives in London and spends a lot of time exploring the city’s parks with her whippet.

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