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What is SMTP?

| 2 min read


Recently, while reading upon mailboxes I was intrigued about the SMTP protocol, and tried to understand how it works.

The Simple Mail Transfer Protocol

The Simple Mail Transfer protocol is responsible for sending email from a sender’s email address to a recepient, by connecting through multiple MTAs in order to find the right mailbox to transfer email to.

Since SMTP is a client-based protocol, it uses DNS to find the right IP to send emails to.

Playing around with a simple SMTP Server

We can use the following python command to create a simple SMTP server on port 2525.

$ sudo python3 -m smtpd -n -c DebuggingServer localhost:2525

The command doesn’t return any output once the server is started.

Now we use telnet to connect to the SMTP server using the command:

$ telnet localhost 2525
Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 pop-os.localdomain Python SMTP proxy version 0.3
250 pop-os.localdomain
250 OK
250 OK
354 End data with <CR><LF>.<CR><LF>
Hello World from SMTP!
250 OK

  • The HELO command is responsible for letting the SMTP server know that an email message is incoming
  • The MAIL FROM command mentions the sender’s mail address.
  • The RCPT TO command mentions the receiver’s mail address.
  • The DATA command refers to ASCII encoded data, that is going to follow it and needs to be terminated by an empty line and a .(dot) which is what the End data with <CR><LF>.<CR><LF> is.
  • Similar to HTTP, 2XX status codes are positive 3xx codes are positive intermediate, 4XX are negative.
  • Particularly 220 refers to <domain> Service Ready, 250 refers to requested mail action okay, completed. The list of all status codes can be found here.