I see a response message “Thank you for your message. It has been sent.” with green border, but I never receive a mail for that.

Showing the green border message means that the PHP function for sending the mail has certainly completed successfully. So if you can’t receive the mail, it’s highly possible that the mail has been kidnapped or killed after that.

If you can check the log of your mail server, it could give you some clues. Spam filter often causes this kind of problem.

See also: Best practice to set up mail

Just another contact form plugin for WordPress. Simple but flexible.