This article explains how to solve the problem when:
1. New user registers and is not sent the activation email
2. Contact form when submitted, returns the error:
SMTP Error! Could not connect to SMTP host.
The issue is that your smtp server is not properly configured in joomla and mail options are wrong
Solution is simple, goto :
joomla administrator > global configuration > server
On that page, to the right side, find this option:
In that set of fields, simple change Mailer to PHP Mail Function
And save the settings.
Test your registration process and contact page once again to confirm.