Ok (I'm going to figure out a way to make a canned response for the forums, just kidding...
) the 90 million dollar questions are:
1. Server OS (web server, not mail)
2. Apache Version
3. Do you have a PHP info file hanging around that I can look at?
4. What version of HCL (after finding so many 1.x.x versions on the net, I gotta ask)
5. New install, or existing?
6. Do you have access to mail server log files?
BTW, thanks for posting the error string, it sent a blank message, which postfix subsequently drops...