Add the "from" address of a Sheldon mail to your addressbook (Contacts) and it should never be flagged as spam.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort.
Solution: Go into your Spam/Junk/Filtered folders, find a Sheldon e-mail and label it as "Not Spam", and that should take care of it. The problem is on GMail's end, unfortunately.
This happens from time-to-time on the bigger mail systems like Yahoo, GMail, or in corporate/government systems: Because their spam algorithms are shared across users, when something flags the Sheldon deliveries as "spam" in their system, then suddenly it's spam for everyone using that system. It's annoying...but not having aggressive, shared spam filters would probably be more annoying in life.
So! If and when you don't get your Sheldon daily e-mail, check your computer-based or server-based spam filters. 99.99% of the time, that's the problem.
(Also: If there are Gmail or Yahoo engineers who'd like to offer up a more permanent solution, that'd be awesome.)
Add the "from" address of a Sheldon mail to your addressbook (Contacts) and it should never be flagged as spam.
I've no GMail, I'm in a Gov office and they don't have filters for Sheldon (they like it) and still haven't received Sheldon since last week. My account is from hotmail and I use LiveMail to read it. I've checked the filters and no Sheldon blockades appear. HELP!!