Confusing Problem With Email Delivery From My Site to AOL / Others

[entropy]

The confirmation emails sent from my website http://earthlandrealms.com when users sign up isn't being delivered to aol.com address's and some other email providers. The strange thing is if I send the same exact email manually using the web based email program Horde, on the same server, using the same email address, it gets delivered. Both Horde and the page sending the email are PHP. The only thing I can think of is that maybe the code is wrong somehow even though the emails do get delivered to gmail and others. Here's the code:

 

$ism=mail($cgi['email'],"EarthlandRealms.com Activation e-mail","Welcome to Earthland Realms! \n Your username is: {$cgi['username']} \n Your activation password is {$pas} \n\nEarthlandRealms.com", "From: [email protected]");