Hi, this is what the hosting company said:
While the mail function isn't strictly disabled, it IS unreliable to send email from the webserver itself. We strongly recommend you switch to a method of sending that allows for remote authentication to your mailserver, like PHPmailer or PEAR. Your plugin developers may be able to provide more information on whether or not their plugins are compatible with these methods.
I don't guess that is very helpful...any other ideas?
Thanks,
Matt