I have a few email accounts with multiple providers so I use the regular old Apple Mail app to access them all in one place.

About a month a go I started getting emails every couple of days from yahoo (legit) telling me I do not have their app and I should get it.
Probably about the time I started getting the emails. The Apple App takes forever to load emails from my yahoo account.
Sometimes up to 30 seconds to load. Just get the little spinning wheel for awhile. Then the email content appears.

There is no such lag with my other accounts accessed with the Apple Mail App.

Yahoo has security features that have to be set
in the mail settings, they want you to enact specific features, it may play havoc with Outlook, and apple mail like on eyeFones.

My office outlook would come up and repeatedly ask for manual logins even though the username/password was in the account setup.

I just dumped them for business email and fixed the problem that way after a year of dealing with it. If I enacted all the security features outlook wouldn't pull the mail consistently..

I had this issue too.

I moved from yah/out to gmail and just use the latter's app. I get enterprise email w/ the apple app and still get the spinning circle a lot. I hate the apple app. Same with the calendar app for enterprise.

I don't think apple plays well with other email/calendar providers.

I went through this some weeks back with my old iPhone 4s, eventually it stopped working.

So I switched it to my work iPhone 6. And all has been well... until a couple days ago when they started sending the same messages.

IMO... the real issue isn't security, it's that since I'm not logging into Yahoo, or using a Yahoo Ap, I'm not seeing any ads. Which is what they want.