PHP mail() function

mail0 is sending mail that can't be received by aol or my local isp. That same mail can be received by other sites on that same server. The mail doesn't bounce but I am getting a few DSN errors in my maillog. My site is hosted at a major hosting company. Funny enough, when the scripts were run on their server w/ php as a cgi, all worked well. When I moved my scripts to a server that had mod_php, I began having problems. Any wizards out their have any ideas? The hosting company seems to have run out of ideas. The PHP version is 4.0.3pl1 running on Redhat 6x with Apache 1.3.14 I have tried various header configs, double checked my php.ini file sendmail config, but after a week of dealing with this, I'll try anything....I'll even dance a jig if that is what it takes. TIA, Kevin