It could be one of two things.. your code might have a problem or the X10 mail server is having a bad day. This does happen on occasion, mail sent using the 'mail() command can take several hours to arrive on occasion but this is thankfully rare.
IF you send the PHP mail to a Email box on the same (x10Hosting free) sever
in a good 24 hrs you will receive about 90% of what you sent
sending four (4) per hour
If you send it off the server via PHP mail, you will receive less