PHP has to be configured correctly on Windows systems to be able to send out mail. If this is not done, you might get errors and mail will not arrive. What you need to do is edit the file "php.ini" (this file holds the configuration for PHP). Find the section that is called "[mail function]". In this section, set the parameter "SMTP" to the hostname or IP address of the your SMTP server. If you do not know your SMTP server, please ask your internet access provider. After this, PHP will know what mailserver to use for sending out mail messages.
TODO: module is bundled with 5.2 If you have no access to the php.ini file and the system administrator is not willing to update it for you, you will have to install the SMTP module for Phorum. You can download this module from the Phorum list of modules. In the settings for this module, you can specify the SMTP to use for sending mail. When using this module, you will completely bypass PHP's built-in mail system.