I have had the same email address and password for years,
it works fine on my Imac and macbook air,it will work fine on my iphone6 for a few weeks but every so often my iphone rejects it.
I have tried deletring the account in settings/mail then resetting it, but it still get the same message
"Cannot get mail"
The username or password for mail.btinternet.com is incorrect.
It is not incorrect. I use it all the time
Read more: Why does my iphone6 not accept my mail password?