Drupal 6 site not sending password reset emails
Are other kinds of mails correctly sent, or are ALL mails from the senong site discarded ? 

If other mails are sent from the same server/site, then something's wrong with the Drupal installation.

If they're not and the sending server's logs are indicating a correct sending, then I'd check the destination server logs to see what's happening here. Sometimes mails got discarded for voodoo reasons, like blacklisting (which is not the same as spam detection) etc.