Does it exist?
Umm, an email library that does what?
Do you mean sending an email using an arduino and ethernet shield?
There are already a few threads with this topic.
Check this:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1235534880
yes, I saw the threads, but they do not talk about an existing library, that's why I'm asking.
You did not describe what you want to do.
You do not always need a library. You can program a sketch without it 
I'm lazy. :
So... what exactly do you want to do?
Of course, sending an email upon an "event" (i.e. specific input).
But aI read about complexity due to authentication and encryption, so I really need a library... :
Link #1: LMGTFY - Let Me Google That For You
You are probably better off using an intermediate server (or proxy). You could have a small script running on a web server that the Arduino sends a request to. The script could then generate and send the email for you. The overhead to form a HTTP request (and send it) is far smaller than communicating with an SMTP server.
Thanks for your help, but if nobody has info about SMTP libraries, I think I' ll be "forced" by moderators to ask in the other threads. 
Well, roll your won lib, you will learn a lot that way.