Send Email Via Telnet Smtp – Let’s take a look at what happens “under the hood” when you send an email. This article covers early implementations of the SMTP protocol based on available research papers and documents. Of course, SMTP has evolved over time, but this article will cover the basic elements of this communication protocol.
We will break down the SMTP role, look at the connection between the SMTP client and the receiving SMTP server, and finally, we will use the Terminal to send emails by issuing SMTP commands directly to the SMTP server.
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If you’ve ever configured an email client before, you may already see terms like IMAP, POP, and SMTP. While IMAP and POP are protocols used for retrieving emails from mailboxes, SMTP is a sending protocol. We will consider IMAP and POP more closely in separate articles.
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SMTP is part of the application layer of the TCP/IP protocol and normally works on port 25. It uses a process called “shop and forward” that is used to organize email sending over different networks. Within the SMTP process, there are small software services called Mail Transfer Agents that help manage the transfer of mail and its final delivery to the recipient’s mailbox. Not only does SMTP define this entire communication channel, but it can support the delivery of delayed mail either to the sender’s site, to the recipient’s site, or to an intermediate server.
TLDR: SMTP describes a communication protocol that describes how e-mail is sent from your computer’s e-mail address, possibly over multiple networks, to an e-mail delivery server on an SMTP server.
User Agent (UA): An application used to send and receive electronic mail (Outlook, Mozilla, etc.) Mail Transfer Agent / Message Transfer Agent (MTA): A method running on an SMTP server that helps send mail by the correct recipient and help manage the delivery of emails to the user’s mailbox. The MTA always helps maintain the mail queue so recovery attempts can be made if the remote server is unavailable. An MTA usually includes a special software called a mail delivery agent or message delivery agent (MDA) that is responsible for delivering email messages to the recipient’s local mailbox, while an MTA is more focused on sending emails. from SMTP servers. to the SMTP server.
When a user sends an email, through a user agent – for example, the Apple mail application – the client’s SMTP protocol opens a TCP connection to an SMTP protocol running on a server somewhere (ie smtp.gmail.com) on port 25. After In the first instance, the SMTP client and server processes a brief response to a request to send an email. We will consider this stream in the near future.
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Take this basic use case of the SMTP protocol (presumably the email will be sent within the same organization):
In reality, however, an email message may pass through several intermediate MTAs (aka. email gateways) en route to delivering the message to the correct recipient.
An additional advantage of email gateways is that they can take emails prepared by other processes and convert them to SMTP format before sending them together or vice versa.
We will note the actual commands and payloads sent between the client and the SMTP server shortly. Before we do that, let’s take a look at how email content is structured.
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An email has two parts – the envelope and the message/body – just like a regular letter.
The envelope contains sender and receiver fields as well as some additional metadata (such as timestamp, MIME properties, etc.)
Let’s see what a modern title looks like. Here are the subject lines of promotional emails from Caviar:
As you can see, it can contain much more than information about the sender and recipient; these points are the minimum requirements.
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Any text that follows the envelope is classified as body. The body is the content of the email generated by the user. The body section usually ends with an empty link, or one of the few characters that can end (ie “.”).
Envelopes are often sent separately from the body of the mail. The SMTP server will first try and verify the sender/recipient information in the headers before considering the authentication and authorization of sending the email body.
These fields are sent to the SMTP server using the MAIL FROM and RCPT TO commands (we’ll take a closer look at these commands).
On the receiving SMTP server, it will periodically check the contents of the mailbox for new mail and forward or forward them if necessary.
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As mentioned earlier, SMTP is part of the application layer of the TCP/IP protocol. As a result, it is not surprising that SMTP uses DNS to determine the IP address of the SMTP server to connect to.
Once the recipient’s IP address is obtained, the client SMTP server can communicate with the remote SMTP server using a short sequence of commands.
HELO – Helps identify the SMTP client protocol to the SMTP server and is always followed by a valid hostname. This is only sent once per session. After this authentication process is completed, the SMTP client process can send as many emails as desired.
MAIL FROM – indicates the SMTP server of the sender’s email address (ie email from site)
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RCPT TO: After the MAIL FROM command, the client’s SMTP server can issue one or more RCPT TO commands to specify the email addresses of all recipients.
DATA: This is the pre-order of the email body. This is the command that tells the SMTP server that it should be prepared to accept ASCII content that ends with a blank line or “.”.
We will see these commands in the application using Terminal soon, but it is important to know that these commands are sent as ASCII text that allows to set up email clients and servers on any platform.
Although the list of SMTP commands is much larger than this list, HELP, mail FROM, RCPT TO, DATA, and QUIT are basic commands.
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After the first handshake is completed (the HELO part), the SMTP session begins. The client sends the email address of each sender and recipient (think of it as an envelope) to the SMTP server. After each client command, the server will respond by sending “250 OK” if the command has been received and verified. Otherwise, it will send an error message, usually prefixed with “550” as the error number.
After receiving confirmation from both sites, the SMTP client process sends the body of the email to the server.
Here’s an example of the whole flow taken from Wikipedia, take a moment to look at it – we’ll extend it via Telnet, Python, and Terminal next:
S:220 smtp.example.com ESMTP Postfix C:HELO relay.example.com S:250 smtp.example.com, nice to meet you C:mail FROM: S:250 Ok C : RCPT TO: S:250 Ok C:RCPT TO: S:250 Ok C:DATA S:354 End with . C: From: “Bob Example” C: To: Alice Example C: Cc: [email protected] C: Date: Wed, 15 Jan 2008 16:02 : 43 -0500 C: Subject: Test Message C: C: Hello Alice. C: This is a test message with 5 headers and 4 lines in the message body. C: Your friend, C: Bob C:. S: 250 Ok: in line for 12345 C: QUIT S: 221 Bye
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We have received 250 confirmations from the server, so we are ready to move on to the next step. Provide envelope header information – sender and recipient.
“354 End with .” tells you that to end the DATA section of the email, type “.” on a new line.
Ordering Note the channel shutdown response 221. Each SMTP command comes with an acknowledgment from the SMTP server.
What we did manually in Terminal is a typical example of the SMTP process in action and what happens, to a large extent, every time you click Send in your email client.
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For this post, I’m using example.com as my email server domain name and email address. Also, I use the default port 25 for SMTP and 110 for POP3.
The first thing you should do is install Telnet if it isn’t already there. (Telnet is not installed by default on all systems for the more secure SSH protocol.) To install Telnet on a Red Hat Enterprise Linux-based system, type:
Now that you’ve installed Telnet, you’re ready to get started. Let’s look at SMTP first
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