How to send an email with Arduino and a WiFi shield?
What you're looking for is a SMTP (Simple Mail Transport Protocol) library. A Google search finds 2 for the Arduino:

SMTPClient
This seems to be a minimal-capability SMTP-only library that piggybacks on top of a connection library.

FullIP
This seems to be a full IP package for the Arduino that works with select network devices which includes SMTP capability.