What is an SMTP Server?

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What is SMTP?

SMTP stands for Simple Mail Transfer Protocol. SMTP is responsible for sending emails from an email client installed on your computer to an email server. An SMTP server receives mail messages sent by users and sends them to the intended recipients of the email.

Sending e-mail messages by using Email Client Software

If you are running an email client like Outlook Express, Mozilla Thunderbird, etc., you would first need to configure the email client to recognize the SMTP server. In case you only use Webmail for your emails, there is no need to configure anything.

How does an SMTP server work?

Whenever you send an e-mail using an email client installed on your computer, it interacts with the SMTP server and ensures that your email is sent to the intended recipients. The SMTP server where your email is hosted may then need to interact with other SMTP servers to ensure that your email is delivered.

SMTP is a relatively simple, text-based protocol. The sender of an email specifies one or more recipients of an email message (and in most cases verified to exist) along with the email text and possibly other encoded objects. When an email is sent from the mail client installed on your computer, this is what happens:

  • Your email client connects to the SMTP server using port 25
  • The mail client interacts with the SMTP server, and coveys to the SMTP server, the addresses of the email sender and receivers along with the contents of the email.
  • The SMTP server then takes the “to” addresses and breaks them into two parts: the “name” of the recipient and the domain name where the recipient has an email account.
  • The SMTP server then interacts with a Domain Name Server, or DNS and gets the IP address of the SMTP server of the receiving domain.
  • The sender’s SMTP server then contacts the receiver’s SMTP server using port 25 and delivers the email to the receiver’s SMTP server. Finally, the receiver’s SMTP server puts the email into the mailbox of the receiver.

If the sender’s SMTP server fails to connect with the receiver’s SMTP, then the email is put in a queue. If the email cannot be sent for four hours, it notifies the sender that there is some sort of problem. If the mail hasn’t been sent from the queue for five days, then the email is returned as a “bounced” mail to the sender.

SMTP Server vs. Pop3 Server

An SMTP server performs two functions:

  • The SMTP server verifies that the configurations are correct and grants permission to the computer trying to send a message.
  • The SMTP server then sends the message to its recipients and tracks the successful delivery of the message.

Each SMTP server has its own unique code by which it is identified. The SMTP code for your mail server may look something like: smtp.yourmailserver.com

SMTP is a “push” protocol that can not “pull” or retrieve messages from a remote server on demand. To retrieve messages only on demand, which is the most common requirement on a single-user computer, a mail client must use POP3 or IMAP.

POP3 is an acronym that stands for “Post Office Protocol version 3”. POP3 enables the downloading of a message sent by any SMTP server. Your mail server would also have a POP3 code that needs to be configured into your mail client. Your mail server’s POP3 code may look something like this: mail.yourmailserver.com


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