Testing services with telnetEdit

Testing SMTP

For added security, I use authenticated SMTP for sending mail, using the LOGIN mechanism. This requires that first I had to Base64 encode my username and password:

perl -MMIME::Base64 -e 'print encode_base64("username");'
perl -MMIME::Base64 -e 'print encode_base64("password");'

For example, to test SMTP you could use something like:

telnet mail.example.com 25
(Base-64 encoded username)
(Base-64 encoded password)
MAIL FROM: <user1@example.com>
RCPT TO: <user2@example.com>
FROM: <user1@example.com>
Mail test, please ignore.

As documented here, to test secure SMTP you would need to use openssl instead of telnet:

openssl s_client -starttls smtp -connect host:port

Where port is most likely 25. In the case of SMTP over SSL (not using STARTTLS) you would use:

openssl s_client -connect host:port

In this latter case port is most likely to be 465.

But note that, as commented here, any line beginning with R will trigger a renegotiation of the SSL/TLS session so you must enter:


And not:


You can also suppress this behavior by passing in the -ign_eof switch.

Alternative host and port syntax

It is possible to pass in the host and port options explicitly:

$ openssl s_client -host smtp.gmail.com -port 587 -starttls smtp

Testing IMAP

This page offers a nice overview.

See also

  • This page has lots of examples on how you can use telnet to manually test various services [February 2010, the link is now dead].