Gmail! But it looks like resetting the app to factory fixed it. Must have been some bug somewhere!
I haven't made the wifi test yet man, cause I need to use my phone today. I plan on making it on Tuesday because I'll be the whole day at work meaning I'll have wifi and my boss won't be looking for me!!!
Ok, my last advice for you on this matter is to just clear the email app data again and forget about it, if you are just using it for Gmail, just use the Gmail app for it, low resources and battery friendly, plus it pushes your emails by default no need to do anything else, in fact, if you haven't disabled Gmail or turned off its sync, then you are using twice the battery and resources just to get the same emails as Gmail account is set by default by simply login with your Google account.
I use most of Samsung Native apps except S Voice and Email, and it's because I don't like S voice and my 2 emails accounts are Gmail accounts so I use the Android native one and that one is Gmail.
After you can make the Airplane mode tests you should have a better picture of what may be wrong in your case, I hope you finally figure it out. Good luck.