First, check if PHP is working on your host
create
phpinfo.php
<? phpinfo(); ?>
now open this file from browser
http://www.geocities.com/youraccount/phpinfo.php
If PHP is working on your site then, the problem might be
in your code
header ("Location: http://www.geocities.com/thankyou.html");
you are redirecting to thankyou.html of geocities... I guess there should be an account name before that... something like
http://www.geocities.com/youraccount/thankyou.html
Error 2:
If the codes(Mail Form and PHP script) are in same file... you sure will get an error.. coz you cant use header() function after displaying something on browser
Check if one of these are your errors
TommYNandA