I recently was fighting with a problem where my PHP emailing scripts would not send emails. The script would run ok (not throw any errors) but no emails would show up to whomever I sent them to. I tried my script on another web host, and it worked perfectly, so I knew something was wrong with my regular host. I questioned them about it to learn that the hostname of my server was not resolveable. It had a name like server1.domainname.com. The support technician changed it to domainname.com and it started working perfectly. Hopefully this will help anybody who has been running into this problem.