I spent hours searching the web trying to figure out why I was getting a "WARNING: mail(): SMTP server response: 501 5.5.4 Invalid Address " every time I was using the mail() function on my server (Win2K3,IIS 6.0,PHP4.4.1). I knew everything was setup properly for SMTP based on other non IIS 6.0 configurations.
Turns out that the IIS 6.0 SMTP service does not like formatting of the "From" field in mail headers. For instance:
<?PHP
//This line DOES NOT send mail message correctly
$headers .= "From: \"".$fromname."\" <".$fromaddress.">\n";
?>
However this works:
<?PHP
//This line sends mail message correctly
$headers .= "From: \"".$fromaddress."\"\n";
?>
The fix is in Microsoft Article ID 291828 (
http://support.microsoft.com/?id=291828 ). Even though the "bug" workaround is for IIS 6.0 on Exchange 2003 communicating with a UNIX server, THIS SOLVES THE PROBLEM. Just skip down to the last section for Exchange 2003 and follow the instructions to modify the IIS 6 MetaBase with the MetaBase Explorer found in the IIS 6 Resource Kit.