Email disappearing on iPhone

My email disappears from my iPhone 7 .  If I open it right after it arrives I can read it, but a few minutes later it will not load, and several minutes after that it disappears altogether and I get a notice that says this message is no longer available.

The only way I know I received most emails is because I can see them on my computer.  Also, when an email first appears on the phone I can move it to another folder I created and I am able to go back and open it later.