For some reason, GMail's filters are handling Sheldon like spam. (Boo, GMail!) To fix that, just do this:

Go into your GMail account, click on "Contacts" (should be on the lower-left side of the screen), and add this e-mail address as one of your "new" contacts:

do_not_reply@sheldoncomics.com

Even Google can be lame, sometimes...but this should fix it.