Post

Send Email using Telnet with TLS and Authentication

Posted
By Sakharam Shinde 2 min read

Send Emails using Telnet

  • We can use telnet to test and debug if a particular machine is able to send e-mail via an SMTP server.
  • Since SendGrid offer 100 free emails everyday, we will send emails using the SendGrid STMP server.

Installation

Operating SystemCommands
Linux - Ubuntu/Debiansudo apt-get update
sudo apt-get install telnet
Linux - CentOS/RHELsudo yum update
sudo yum install telnet
Linux - Fedorasudo dnf update
sudo dnf install telnet
WindowsAdd-WindowsCapability -Online -Name Telnet.Client~~~~0.0.1.0
MacOSbrew install telnet

Telnet Commands to send an email:

Step 1. Connecting to the SMTP server (SendGrid)

PortCommand
SMTP - 25telnet smtp.domain.com 25
TLS - 587openssl s_client -starttls smtp -connect smtp.sendgrid.com:587
 openssl s_client -starttls smtp -ign_eof -crlf -connect smtp.sendgrid.com:587
SSL - 465openssl s_client -connect smtp.sendgrid.com:465
  • If the connection is established, we should the similar response as below:
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    Trying 159.122.219.43…
Connected to smtp.sendgrid.net.
Escape character is ‘^]’.
220 SG ESMTP service ready at ismtpd0001p1lon1.sendgrid.net

Please note that you might see additional information for SSL and TLS connections.

Step 2. Initiate the conversation

  • Type the below Extended Hello (EHLO) command with the domain name or IP address to initiate a conversation:
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      EHLO domain.com
  • The SMTP server will respond with the SMTP Service Extension list which includes supported capabilities and service extensions.
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      250-smtp.sendgrid.net
  250-8BITMIME
  250-PIPELINING
  250-SIZE 31457280
  250-AUTH PLAIN LOGIN

Step 3: Authentication

  • Before we authenticate to the server, the username and password needs to be converted into base64 format:
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      $ echo -ne "username" | base64
  dXNlcm5hbWU=
  $ echo -ne "password" | base64
  cGFzc3dvcmQ=
  1. Using PLAIN The PLAIN mechanism expects a base64 encoded string containing both username and password, each prefixed with a NULL byte. To generate this string using the base64 binary, run this command:
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     $ echo -ne "\0username\0password" | base64
 AHVzZXJuYW1lAHBhc3N3b3Jk
 AUTH PLAIN AHVzZXJuYW1lAHBhc3N3b3Jk
 235 Authentication successful
  2. Using AUTH LOGIN
    • You will be prompted for the username first, then the password. The entire conversation will look like this:
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     334 VXNlcm5hbWU6
 dXNlcm5hbWU=
 334 UGFzc3dvcmQ6
 cGFzc3dvcmQ=
 235 Authentication successful

Step4: Send the E-mail

  • Using the below commands, send an email: a. The sender (MAIL FROM) b. The recipient (RCPT TO) c. The message body (DATA)

  • Once we are ready to send our message, we end with a single dot (.) character.

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      MAIL FROM: john@doe.com
  250 Sender address accepted
  rcpt to: mary@jane.com
  250 Recipient address accepted
  DATA
  354 Continue
  From: from@domain.com
  Subject: Test message!

  Hi,

  This is a test message!

  Best,
  John
  .
  250 Ok: queued as bazLUK4DEBqH25dH6iZuNg

Note: if you connected with openssl instead of telnet, you have to make sure to type the rcpt to command in lowercase. Pressing R in the client session instructs openssl to renegotiate the TLS connection.

Step 5: End the session

  • Type QUIT to close the session.
Exchange
Exchange Telnet
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