Just confirming what seems to be the new norm, unless I'm way off, but it would appear that you can no longer set a Custom Check Interval of less than five minutes with newer Android versions. In my case that would be Nougat 7.0. And this is a limitation of the OS and its allowance of background tasks that are too frequent?
I don't normally need such a short interval (normally I fetch my two Yahoo accounts at 10 min. intervals).
But sometimes recently I've gotten into quicker back and forth conversations on email that I could appreciate use of a shorter interval.
Not a big deal. That's what IM chat apps are for! I just have to remember to negotiate both of us over to one we both have !