Connecting to smtp.gmail.com via command line
Using Linux or OSx, do what Sorin recommended but use port 465 instead. 25 is the generic SMTP port, but not what GMail uses. Also, I don't believe you want to use -starttls smtp
openssl s_client -connect smtp.gmail.com:465
You should get lots of information on the SSL session and the response:
220 mx.google.com ...
Type in
HELO smtp.gmail.com
and you'll receive:
250 mx.google.com at your service
From there it is not quite as straightforward as just sending SMTP messages because Gmail has protections in place to ensure you only send emails appearing to be from accounts that actually belong to you. Instead of typing in "Helo", use "Ehlo". I don't know much about SMTP so I cannot explain the difference, and don't have time to research much. Perhaps someone with more knowledge can explain.
Then, type "auth login" and you will receive the following:
334 VXNlcm5hbWU6
This is essentially the word "Username" encoded in Base 64. Using a Base 64 encoder such as this one, encode your user name and enter it. Do the same for your password, which is requested next. You should see:
235 2.7.0 Accepted
And that's it, you're logged in.
There is one more oddity to overcome if you're using OSx or Linux terminals. Just pressing the "ENTER" key does not apparently result in a CRLF which SMTP needs to end a message. You have to use "CTRL+V+ENTER". So, this should look like the following:
^M
.^M
250 2.0.0 OK
For OSX' terminal:
openssl s_client -connect smtp.gmail.com:25 -starttls smtp
Start session from terminal:
openssl s_client -connect smtp.gmail.com:25 -starttls smtp
The last line of the response should be "250 SMTPUTF8"
Initiate login
auth login
This should return "334 VXNlcm5hbWU6".
Type username
Type your username in base64 encoding (eg. echo -n 'your-username' | base64)
This should return "334 UGFzc3dvcmQ6"
Type password
Type your password in base64 encoding (eg. echo -n 'your-password' | base64)
Success
You should see "235 2.7.0 Accepted" and you're are successfully logged in