Someone asked me a while back about authenticating to send email, and I can't find the thread. My apology for not remembering who you are.
I just tested an email client using SMTP2GO, and it works fine. It is the second sketch on this playground page.
http://playground.arduino.cc/Code/Email
It requires base64 encoded user and password, which you can do here.
Use the UTF-8 character set.
edit: Here is the serial monitor output.
Ready. Press 'e' to send.
connected
220 smtpcorp.com ESMTP Exim 4.85 Thu, 07 May 2015 21:39:28 +0000
Sending hello
250-smtpcorp.com Hello [1.2.3.4]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH CRAM-MD5 PLAIN LOGIN
250-STARTTLS
250 HELP
Sending auth login
334 VXNlcm5hbWU6
Sending User
334 UGFzc3dvcmQ6
Sending Password
235 Authentication succeeded
Sending From
250 OK
Sending To
250 Accepted you@yourdomain.com
Sending DATA
354 Enter message, ending with "." on a line by itself
Sending email
250 OK id=1YqTVw-4gfMh7-KO
Sending QUIT
221 smtpcorp.com closing connection
disconnected
Email sent
