timster - 12:56 am on Mar 4, 2011 (gmt 0)
Sounds like your host is blocking outgoing mail deliberately. (You can try testing your script on another host to be sure the script is not the problem.)
The most common way a host blocks outgoing mail is to block traffic on port 25. You may be able to get around this by finding a web host that accepts SMTP connections on port 26 (or some other port).
You might also Google "php port scanner" to test if your host is indeed blocking your port 25.