Sending Email From Smtp Server – Let’s take a closer look at what happens “under the hood” when you send an email. This article covers the original implementation of the SMTP protocol based on available research articles and documentation. Of course, SMTP has evolved over time, but this article will cover the basic elements of this communication protocol.
We’ll break down the responsibilities of SMTP, examine the communication between the client’s SMTP server and the receiver’s SMTP server, and finally use the terminal to send email by issuing SMTP commands directly to the SMTP server.
Sending Email From Smtp Server
If you’ve ever set up an email client, you may have seen terms like IMAP, POP, and SMTP. While IMAP and POP are protocols used to retrieve email from a mailbox, SMTP is purely a sending protocol. Let’s take a closer look at IMAP and POP in a separate article.
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SMTP is part of the application layer of the TCP/IP protocol and traditionally runs on port 25. It uses a process called “store and forward” that is used to orchestrate the sending of your email across different networks. Within the SMTP protocol, there are smaller software services called mail transfer agents that help manage the transfer of email and its final delivery to the recipient’s mailbox. Not only does SMTP define the entire communication flow, it can also support delayed delivery of e-mail at either the sender’s site, the recipient’s site, or any intermediate server.
TLDR: SMTP defines a communication protocol that dictates how e-mail travels from your computer’s mail transfer agent, possibly across multiple networks, to the mail transfer agent on the destination SMTP server.
User Agent (UA): an application used to send and receive e-mail (Outlook, Mozilla, etc.) Mail Transfer Agent / Message Transfer Agent (MTA): a process running on an SMTP server that helps deliver e-mail messages to the correct recipient and helps manage e-mail delivery to the inbox of the user. MTAs often help maintain the mail queue so that redelivery attempts can be scheduled in the event that the remote server is unavailable. An MTA often contains special software called a mail delivery agent or message delivery agent (MDA) that is responsible for delivering email to a local recipient’s mailbox, while an MTA focuses more on delivering email from an SMTP server. to the SMTP server.
When a user sends email through a user agent – such as Apple’s Mail app – the SMTP client process opens a TCP connection to an SMTP process running somewhere on the server (ie smtp.gmail.com) on port 25. After the initial handshake, the SMTP client and server processes participate in a mailbox -A short request-response dialog for sending an e-mail. We’ll take a closer look at this flow soon.
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Let’s take this basic example of using the SMTP protocol (this situation is probably a situation where e-mail is sent within the same organization):
In reality, however, an email message may pass through several intermediate MTAs (known as mail gateways) en route to delivering the message to the correct recipient.
Another advantage of e-mail gateways is that they can handle e-mail prepared using another protocol and convert it to SMTP format before forwarding it or vice versa.
We’ll take a closer look at the actual commands and payloads sent between the client and server SMTP processes shortly. Before we do that, let’s take a look at how email content is structured.
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An email consists of two parts – the envelope and the message/body – just like regular mail.
The envelope contains sender and recipient fields and some additional metadata (such as timestamp, MIME properties, etc.). Fields in the envelope section are often formatted like this:
Let’s see what modern heads look like. Here are the email headers for Caviar’s sales promotion:
As you can see, header fields can contain much more than sender and recipient information; These fields are a minimum requirement only.
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Any text after the envelope is classified as body. The body is the user-generated content of an email. The body section is usually terminated by a blank hyphen, a blank line, or one of several possible termination characters (ie, “.”).
The envelope is usually sent separately from the body of the email. SMTP servers will first try to verify the sender/recipient information in the header before they consider and bother to transmit the body of the email.
These fields are sent to the SMTP server using the MAIL FROM and RCPT TO commands (soon we will take a closer look at these commands).
It will periodically check the contents of its mailbox on the receiving SMTP server for new e-mail messages and forward or forward them as 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 being addressed.
After receiving the recipient’s IP address, the client’s SMTP server can connect to the remote SMTP server using a short sequence of sequential commands.
HELO – Helps identify the SMTP client process to the SMTP server, usually followed by a fully qualified hostname. This is usually only sent once per session. After completing this authentication step, the SMTP client process can send any number of e-mail messages.
MAIL FROM – Identifies the sender’s email address for the SMTP server process (ie the sender field in the email message)
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RCPT TO: After the MAIL FROM command, an SMTP client server can issue one or more RCPT TO commands to specify the e-mail addresses of all recipients.
DATA: This is the command that comes before the email body. This is a command that tells the SMTP server to prepare to receive ASCII content that ends with a null or “.” line.
We’ll soon see these commands in action using the terminal, but it’s important to note that these commands are sent as ASCII text, making it possible to build mail clients and servers on any platform.
Although the list of SMTP commands is larger than this list, the main commands are HELP, MAIL FROM, RCPT TO, DATA, and QUIT.
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After the initial handshake (HELO phase) is completed, the SMTP session begins. The client first passes the sender and recipient email addresses separately to the SMTP server (think of an envelope). After each command from the client, the server will respond by sending “250 OK” if it received and confirmed the command. Otherwise, it will send an error message, usually prefixed with “550” as the error number.
After receiving confirmation of these two fields, the SMTP client process forwards the body of the email to the server.
Here’s an example of the entire flow taken from Wikipedia, take a moment to review it – next time we’ll simulate the exact flow using Telnet, Python and Terminal:
S: 220 smtp.example.com ESMTP Postfix C: HELLO 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 data with . C : From: “Bob Primer” C: To: Alice Primer C: C: [email protected] C: Date: Tue, 15 Jan 2008 16:02: 43 -0500 C: Subject: Test Message C: C: Hello Alice. C: This is a test message with 5 header fields and 4 lines in the message body. C: Your friend, C: Bob C: . S: 250 OK: queue 12345 C: OPEN S: 221 bye
Sending Email With Smtp
We got a 250 confirmation from the server, so we are ready for the next step. Providing information on the envelope title – sender and recipient.
“354 End data with .” tells you to end the DATA section of the email by typing “.” New line.
Command. Note the service response 221, which closes the transmission channel. Each SMTP command is accompanied by a confirmation from the SMTP server.
What we just went through manually in the terminal is a typical example of the SMTP protocol in action and what happens on a much larger scale every time you press send in your email client.
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I hope you enjoyed this deep dive into the SMTP protocol. If there are any other algorithms or technologies you’d like us to unpack, leave a comment below! The postmark can be used to send email from Outlook using SMTP without affecting the ability to receive email to an email address using an existing IMAP configuration. In this handy article, we show you how to configure the Outlook desktop client so that outgoing email from your Outlook account is sent using a postmark. This article makes useful use of Outlook for Mac 2016 for screenshots. The exact views may be different for other versions of Outlook.
This will display your Outlook accounts. Select the account you want to set up to use the postmark for shipping.
For the outgoing server, enter smtp.. set the port to 587 or 2525 and check Override default port and Use SSL to connect if not already checked.
Click More Options… Grab the Postmark Server API Token
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