Based on this guide I am trying to send a test email using telnet from linux
https://linuxconfig.org/send-an-email-using-telnet
but the connection immediately disconnects:
$ telnet smtp.gmail.com 465
Trying 108.177.126.108...
Connected to smtp.gmail.com.
Escape character is '^]'.
MAIL FROM: asdasd@asd.Connection closed by foreign host
How do I keep the connection open long enough to send my test mail?
Connections to smtp (25) start as unencrypted.
Connections to smtps (465) start/negotiate encryption before any SMTP protocol level communication.
You should get "SMTP greeting message" from SMTP server before sending any SMTP commands.
Classic/standard telnet does not support encryption (ssl - Secure Socket Layer).
You may check if your telnet program supports it.
Package telnet-ssl provides telnet variant with ssl support. It supports command line like below:
telnet-ssl -z ssl smtp.gmail.com 465
One on a few alternatives is provided by gnutls-cli program from gnutls-bin Debian package.
gnutls-cli -p 465 smtp.gmail.com
Since this Q&A came up again on the front page
The canonical tool (almost always already installed as well) is OpenSSL
The relevant sub command to test both explicit TLS / SSL as well as opportunistic TLS SSL with startssl / starttls is openssl s_client
openssl s_client -connect servername:465
And for opportunistic TLS on for example port 25
openssl s_client -connect -starttls smtp servername:25
