I get an Email notification for topics and posts, but not for registration. why is that?
Well registrations are handled by Wordpress. Is that a new issue?
yes, but why the user's part is not being received?
Logins and registrations are handled by Wordpress. What you see in wpForo is a form only.
Posts and Topics emails are queued by wpForo to Wordpress to send.
On how to debug email send issues (smtp), a smtp plugin with logging will help you see what happens.
Make sure you enable (extended?) logs to see what happens.
Is the email initiated, delivered, rejected, whatever.