HTML Formatting lost from XServe

I’m trying to move a site from a linux box to an XServe running 10.4.11
A previously well behaved php script that sends a multipart/alternative message to selected recipients now results in the raw code being seen on delivery but only for some recipients rather than two alternative text and HTML versions The script has been working fine for 5 years and the only difference is the move to the apple box.

In case it matters I initially gaffed setting the server up and had messages going out with a return path to www@localhost.localhost and got blacklisted for some but not all recipients by Spamhaus. I’ve corrected that and gotten de-listed but I think the ISP’s that used the Spamhaus filter are also the ones now not showing the HTML.

Also I have no mail service running on the XServe as I currently just want it as a webserver.

Might this be recipient ISP dependent, stripping something form the header perhaps? And if so is there some other setting in postfix’s main.cf I should be tweaking to get around it.

Thanks for your thoughts.