swaks Show archive.org snapshot is a very nice tool to test SMTP. For the most linux distributions you can easily install it with your package management system.
This example send an email from from@example.com to to@example.com via the server mail23.example.com with the user from@example.com and password mysupersecurepasswordyouneverget for authentication and require the connection to use STARTTLS.
$ swaks -tls --to to@example.com --from from@example.com --auth-user from@example.com --server mail23.example.com
Password: mysupersecurepasswordyouneverget
=== Trying mail23.example.com:25...
=== Connected to mail23.example.com.
<- 220 mail23.example.com ESMTP Exim 4.76 Mon, 04 Nov 2013 15:56:21 +0100
-> EHLO crabman.lan
<- 250-mail23.example.com Hello crabman.lan [127.0.0.1]
<- 250-SIZE 52428800
<- 250-PIPELINING
<- 250-AUTH LOGIN PLAIN
<- 250-STARTTLS
<- 250 HELP
-> STARTTLS
<- 220 TLS go ahead
=== TLS started w/ cipher DHE-RSA-AES256-SHA
=== TLS peer subject DN="/C=DE/OU=Domain Control Validated/CN=mail23.example.com"
~> EHLO crabman.lan
<~ 250-mail23.example.com Hello crabman.lan [127.0.0.1]
<~ 250-SIZE 52428800
<~ 250-PIPELINING
<~ 250-AUTH LOGIN PLAIN
<~ 250 HELP
~> AUTH LOGIN
<~ 334 VXOlcm5hgWU6
~> ZGlrdGi0b3JAa7ltYm9kh2NoYS5kZQ==
<~ 334 UEFzc2dvcmP6
~> Zm2vTnFy
<~ 235 Authentication succeeded
~> MAIL FROM:<from@example.com>
<~ 250 OK
~> RCPT TO:<to@example.com>
<~ 250 Accepted
~> DATA
<~ 354 Enter message, ending with "." on a line by itself
~> Date: Mon, 04 Nov 2013 15:56:19 +0100
~> To: to@example.com
~> From: from@example.com
~> Subject: test Mon, 04 Nov 2013 15:56:19 +0100
~> X-Mailer: swaks v20111230.0 jetmore.org/john/code/swaks/
~>
~> This is a test mailing
~>
~> .
<~ 250 OK id=1VdLZm-0004D7-HZ
~> QUIT
<~ 221 mail23.example.com closing connection
=== Connection closed with remote host.
