You haven't provided much information for us to work with, but the usual problem is that the form/script is trying to use the user's email address as the "from" address on the email. x10Hosting doesn't allow "spoofing" of the "From" header; all mail sent from an x10Hosting account must come from an email account belonging to the web account. If you need to be able to reply to the user, you can use the "ReplyTo" header for the user's email address.