Error With ContactForm Plugin - Internal Server Error
Craft's email SMTP settings are configured under Settings->Email in the control panel.

The error message you're getting:

2015/12/03 19:21:27 [error] [exception.phpmailerException] exception 'phpmailerException' with message 'SMTP connect() failed.


Simply means that PHP/Craft can't connect (for whatever reason) to the server you specified with whatever SMTP settings you've entered in Settings->Email.

Chances are the credentials are invalid or the settings are incorrect, but it can also happen is something (firewall, router, etc.) is blocking output SMTP access as well.