When you send email with a PHP script, you will need to make sure that the "From:" address in your email header is set to an address within your domain name. If the address is set to an external email address, then you will see an error in your /tmp/mailError.log file, and your email will not be sent. For example:
2003 Jun 18 21:18:15 Result: 9
2003 Jun 18 21:18:15 From address not in member domain. Message not sent.
The PHP mail() function is limited to 250 emails per day. Once you have reached this limit, the email generated by your scripts will not be sent until the limit has been reset.
Please note that these emails will not be held for delivery, so you may need to regenerate any emails not sent if your script has reached the 250-per-day email limit.