I installed a new WP site on local machine with wamp server.
When a new user registers it gets the "registration successful" message but a mail with the password is never sent. Also checked the junk folder.
What can be the cause?
Probably your local setup since Wamp does NOT come with mail server. But you can configure the STMP settings of PHP to point to another server in your LAN which runs an SMTP server, or to the SMTP server of your ISP.
this thread has an example of how to set it up: http://www.wampserver.com/phorum/read.php?2,31302,70969
Chances are that this isn't really a WordPress issue, but actually an problem with how you're setup for mail on your local machine / dev environment.
You can install a 'dummy' SMTP server on your local dev machine that allows you some independence from your environment. I often work at client sites where access to port 25 is tightly controlled, and using MockSMTP is very helpful for testing notification emails
For OS X: MockSMTP
For Windows: smtp4dev