Strange mail() behaviors

I've been having this problem for quite a while, but it only occurs on 
one of my servers.

Each server is running the same setup of Postfix, Amavisd-new and Apache.

The two servers that use the mail() function correctly are using 
different versions of PHP (4.3.11 and 5) and the one that is not working 
is using PHP4.3.11.

All are running FreeBSD 5.2 or better.

Whenever the mail() function sends an email on the problematic machine 
it sends it with a "return-path" to "www".

I have not been able to find anything online that would solve this:

"This is the Postfix program at host tsunami.cwis.biz.

I'm sorry to have to inform you that your message could not be
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                        The Postfix program

<TPHAN-EDenNLDtpFT2fBVCVOL8/A@xxxxxxxxxxxxxxxx>: host 
mm.stthomas.edu[140.209.3.222] said: 553 badly
   formatted address - <www> (in reply to MAIL FROM command)

*Failed delivery status*
*Final recipient*       TPHAN-EDenNLDtpFT2fBVCVOL8/A@xxxxxxxxxxxxxxxx
*Reason for failure*    host mm.stthomas.edu[140.209.3.222] said: 553 badly
*Reporting mail server*         tsunami.cwis.biz

"

I am completely lost. I don't know where to look anymore let alone what 
to look for. I hope you can help.

TIA,
Ryan C

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