Should not check email existing if email domain does not have suffix

For the emails like test@test are should not be considered as a valid email and the API request for email checking should not be sent.

Screenshots

Upstream	JiHu

How to reproduce

1. Goto registration page
2. Fill email field with test@test
3. You can see the message is different between upstream and JiHu

Related to #1541 (closed)