Email messages disappear! *poof*

I am having a problem where emails will just *poof* disappear in front of my eyes. I can see a message in my inbox, then it will just be gone. I can reading a message, and it will disappear and take me back to the inbox. It seems to happen mostly with messages that could be confused as junk mail, that are from online stores, mostly. However, it can also happen with messages that are from an institution, like a school or university. The messages are not being moved to Junk or to Trash. The only email account I use in my iCloud/me/mac account that I have had for almost ten years. It was happening on my old iPhone 5S, and is still happening on the new iPhone 6S I got a couple weeks ago. I am not using any other accounts, like POP. I have checked the Junk mail settings on my MacBook Air, and it is set to mark mail as junk, but keep in my inbox. Please help!

iPhone 6S 64GB

Capacity 55.6 GB

Verizon

Version 9.2.1 (13D15)

Model MKT02LL/A

MacBook Air (13-inch, Early 2015)

Processor 2.2 GHz Intel Core i7

Memory 8GB 1600 MHx DDR3

Graphics Intel HD Graphics 6000 1536 MB

500GB Flash Storage