The Galaxy S4 will not be able to get the update, due to the lack of software and hardware on the device. The only highest update this device should be able to receive is android 5.1. If its rooted it will be able to get the 6.0 update once rom developers start encoding it. Then again don't take my word on it. Samsung works in mysterious ways, when it comes to their device updates.
i think these people are moving too fast , they need to fix their previous OS before releasing the next one.
Maybe the new huge update is the fix of the previous version? You think that Apple's OS is 100% bug-free?
I'm also not having any issuies with emails. What kind of issues you have and with which app? Maybe the problems are in your mailaccount or settings.