I would appreciate the Mods getting invovled again. I don't have problems with my other email accounts in the default iOS mail app.
This problem first occurred last year and it won't be resolved until BT/Yahoo recognised the issue and do something about it.
Spouse set up new iPad mini yesterday. 1st thing it did was update to iOS 11.2.5. Exisiting iPad connected to BTYahoo account was OK yesterday. Emails were delivered to both devices at about 7am today. Soon after that both devices failed to deliver. This was before existing older iPad updated to iOS 11.2.5 at about 10am.
iPhone email is functioning OK as normal... no interruption to the service.
We've looked at the settings on BTYahoo webmail but can't find any button to click to allow external connections - a suggested fix.
Logged into BTYahoo webmail (Andy N's suggestion) on the older iPad .... emails very soon started coming to that, and a few minutes later to the new iPad mini.