Update 12/05/2013: This problem is now solved. We are now able to send email to all provider. But still, I would recommend Gmail.com & Live.com.
I found out that our server cannot send emails to Yahoo! (incl. @yahoo.com, ymail.com, rocketmail.com, etc.). I also tried to contact Yahoo! Support but they won’t get this sorted.
So if you are about to register @comiz.net, please do not use Yahoo! Mail service!
Gmail.com & Live.com are more convenient & much more faster!