First connecto to the server. It’s usualy on port 25 but this can change from server to server:

telnet server.com 25

Then say hello:

EHLO server.com

Say you will be providing plain auth:

AUTH LOGIN

The server will ask you for your username (it replies 334 UGFzc3dvcmQ6 which is 334 Username: encoded in base64). Then give you base64 encoded user name. You can use this web to encode it: http://www.base64encode.org/

dXNlcm5hbWU=

Now the server will ask you for your password (it replies 334 UGFzc3dvcmQ6 which is 334 Password: encoded in base64). Give your base64 encoded password:

cGFzc3dvcmQ=

It should anser something like this:

235 Authentication succeeded

Now send the email as usual:

MAIL FROM: 
RCPT TO: 
DATA
Subject: Hello

Your message
.

Note: If you supply an email subject you have to press enter twice before starting with the actual message.

This is how it should look everything together:

$ telnet server.com 25
Trying 97.53.12.9...
Connected to server.com.
Escape character is '^]'.
220-server.com ESMTP Exim 4.80 #2 Wed, 12 Feb 2014 13:37:51 +0100 
220-We do not authorize the use of this system to transport unsolicited, 
220 and/or bulk e-mail.
EHLO server.com
250-server.com Hello 32.92-10-25.blabla.tururu.com [92.10.25.32]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP
AUTH LOGIN
334 VXNlcm5hbWU6
dXNlcm5hbWU=
334 UGFzc3dvcmQ6
cGFzc3dvcmQ=
235 Authentication succeeded
MAIL FROM: 
250 OK
RCPT TO: 
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
SUBJECT: Hi!

Test
.
250 OK id=1WDZ6R-0008KT-Nx

Ref: http://www.ndchost.com/wiki/mail/test-smtp-auth-telnet