I have this same problem from day one and have been sent from pillar to post by Apple. After resetting wi-fi on the phone, doing two factory resets, two trips to the so called ‘genius’ bar, I still have the problem
After being told if the factory reset without using my iTunes backup and manually setting up the Yahoo accounts meant it must be a hardware problem, I went back and they refused to change the phone. On the phone with Apple they finally agreed to change the phone for a used/second hand/factory refurbished phone (they refused to say which, just that it was not new) which arrived yesterday. Another half day lost setting it up and the email worked great. For a few hours. This morning, no new emails. The X said it updated at 07:52, I turn on my 5 (not s, just 5) and there are new emails, same on the iPad mini
So I’ve paid $1,100 for a second hand phone and the problem is STILL there. What a rip-off. DON’T exchange your phone as Apple are ripping people off with non-new replacements. Apple used to be about customer service but now they just seem to be all about profit. I still have my iPhone 3, 5, I have AppleTV, iPad, etc. but I will never buy an Apple product ever again. If Apple had not deluberately crippled my iPhone 5 I would not have changed it. Now I am stuck with a used iPhone X that still does not work properly and am out thousands in lost time, just under five working days