Email delivery discrepancy: Parent vs. subdomain

This was posted originally here >>>http://drupal.org/node/293930

But I realized belatedly that it might be better to address this to those with multisite experience.

I have drupal 6.3 installed and run about 20 other TLDs (site1.com, site2.com, etc., all folders located in the /sites directory) off that single installation. Everything works fine, with one exception.

I've noticed that the standard php mail functions (for user registration and webforms, for example) work for the parent installation, but uniformly do not work for the child subdomain installations.

Is this a common problem, and if so has anyone come up with a solution? I've tried the SMTP module but it did not address the problem.

Thanks.