More actions
The Old Way
To connect to a server using Telnet you would run the following command.
telnet mail.example.com 25
However, you're not going to get very far. Unencrypted Telnet is a no-no for most mail servers and they'll cut you off right there.
Using OpenSSL to Connect Securely
Luckily, you can get around this by using the wonderful openssl tool/library to wrap your telnet session in a warm security blanket weaved from the finest encryption the transport security layer has to offer.
- To connect to a mail server using
openssl:
openssl s_client -connect mail.example.com:25 -starttls smtp -no_ssl3
- Next say hello with your domain name:
helo DOMAIN.COM
- To send a message:
mail from: from@example.com
- Enter the recipient:
rcpt to: to@example.com
You may not be able to send to anyone except for yourself this way because most mail providers will not allow you to send without first authenticating. You'll know you're not allow to send externally because you'll get a 554 5.7.1 Relay access denied error.
- Type data to begin composing your message and . to end
data 354 End data with <CR><LF>.<CR><LF> Subject: Test Email with telnet Hey me, I was able to send an email with telnet .
Authentication
- To authenticate before sending you'll need to run:
AUTH PLAIN
- Then you'll need to enter a base64 encoded string version of your username and password:
USERNAMEUSERNAMEPASSWORD (in base64)
- Then you'll be authenticated to send:
235 2.7.0 Authentication successful
You can generate the base64 encoded username/password string with the following:
echo -ne "USER@DOMAIN.TLD\x00USER@DOMAIN.TLD\x00PASSWORD" | base64 --wrap=0
Where USER@DOMAIN.TLD is your Email address and PASSWORD is your mailbox password.
You can also find the correct base64 encoding of the "username\x00username\x00password" string is by grabbing it out of the verbose output of the SMTP handshake that happens when you're Sending an Email with Curl. This form of SASL PLAIN auth string will always be "username\x00username\x00password" as described in RFC4616
PLAIN SASL Mechanism The mechanism consists of a single message, a string of [UTF-8] encoded [Unicode] characters, from the client to the server. The client presents the authorization identity (identity to act as), followed by a NUL (U+0000) character, followed by the authentication identity (identity whose password will be used), followed by a NUL (U+0000) character, followed by the clear-text password.
Sources
https://hostpapasupport.com/view-send-email-using-telnet/
https://michlstechblog.info/blog/mail-connect-tls-encrypted-to-a-smtp-server-by-telnet/