Email being blocked

Hi, I have a very frustrating problem that I hope someone can give me an answer to. I'm trying to send out emails from a script, using the mail() function. This works, but only for email addresses that are in the same domain as the website ! So, if the website domain that the script is running within is www.mysite.com, then test@mysite.com is fine test@someothersite.com will not go The script is running on a Cobalt RaQ, Linux machine, with sendmail. Accessing sendmail from perl CGI presents no such difficulties, so I assume its not a sendmail / firewall / OS problem. I have also tried talking to sendmail directly, from PHP, rather than using the mail() function, and have found that the same problem occurrs. Could it be something in PHP.INI ? Please help ! Thanks, Ben Peters