UPDATE: Removing an account and readding it works. Give it a go, then restart both applications (itunes and mail.app).
I had a developer preview test installed (Mountain Lion DP3), and downgraded to OSX 10.7 via changing a system file with the version. This is when I began noticing this issue. I have heard there are issues with the maoil app, so I went and restored the file as directed from an old time machine backup. No dice. Not sure if it's related, but wanted to throw it out there. I have sinced downgraded to 10.7 per another post on being able to check the box but no accounts. STILL NO DICE. I cannot check the box for syncing mail.