If the destination is either Hotmail or Yahoo, those sites have blacklisted x10hosting's mailserver and bounce back any emails (they do not even make it to the 'spam folder'). By going to your Webmail for your x10hosting account, you should see the delivery failure notices.
Google does accept the emails, but sometimes they go to the spam folder.
One alternative is to open a Google Apps account for your domain and then use SMTP to send your emails out via Google.