Send Email using Telnet with TLS and Authentication

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 System	Commands
Linux - Ubuntu/Debian	sudo apt-get update sudo apt-get install telnet
Linux - CentOS/RHEL	sudo yum update sudo yum install telnet
Linux - Fedora	sudo dnf update sudo dnf install telnet
Windows	Add-WindowsCapability -Online -Name Telnet.Client~~~~0.0.1.0
MacOS	brew install telnet

Telnet Commands to send an email:

Step 1. Connecting to the SMTP server (SendGrid)

Port	Command
SMTP - 25	telnet smtp.domain.com 25
TLS - 587	openssl s_client -starttls smtp -connect smtp.sendgrid.com:587
	openssl s_client -starttls smtp -ign_eof -crlf -connect smtp.sendgrid.com:587
SSL - 465	openssl s_client -connect smtp.sendgrid.com:465

• If the connection is established, we should the similar response as below:

  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:

    EHLO domain.com
  

• The SMTP server will respond with the SMTP Service Extension list which includes supported capabilities and service extensions.

    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:

    $ 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:

    $ 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:

    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.

    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.