Qmail; PHP 401 and 410

We have several NT 4 based web servers (running IIS) which run the CGI version of PHP 4.0.1. Our SMTP server is a linux box running QMail. 4.0.1 works fine with this QMail server for Mail() command. I've tried to upgrade one server to PHP 4.0.5, 4.0.6 and 4.1.0 - each time I have been defeated by the same problem/glitch/headache: Mail() stops working in that the Mail command never completes and eventually the script times out. If we change the SMTP ini setting to another server (in this case a NT SMTP server which is not configured to send properly, so can't be used as relay and thus not a real solution) the Mail command works again. We have tried a number of different SMTP servers, but we appear to have reached the conclusion that while PHP 4.0.1 is happy to talk to Qmail, later versions are not. Additionally I have also tried installing Apache 1.3.9 and PHP 4.0.1 (then 4.0.5) on my own NT 4 workstation, the former works fine the latter not. I would welcome any thoughts/assistance on this! Thanks... Lee / Emile