Email sent to a non-existent domain

Hi, recently a user could register to my forum using the mistyped mail domain "otmail.com", which should have been "hotmail.com". The mail bounced back with the error "Sorry, I wasn't able to establish an SMTP connection". Why did it pass the forum email check? Perhaps cause the domain "otmail.com" exists (it redirects to "live.com")? Anyway I found a simply script using the "checkdnsrr" function that tells correctly that the email host is not valid. Thanks in advance.