It's not BT Openreach, that doesn't exist. BT (Retail) and Openreach are completely separate companies.
BT email, whether BTYahoo or BTMail is supported for apps/clients - it's just that the helpdesks are unable to offer any help with the wide variety there is out there. They provide the settings to use.
It's true the helpdesk, including Twitter, say if webmail is working then all's well. However, regulars in here know that to be a complete load of cobblers.
Gmail can, of course, can easily be used by an app/client - negating any need to see adverts per se
Thanks. I'm going to slowly shift my email address away from BT. Although I'm not a fan of gmail for other reasons. Problem is I also have other email accounts from another country and those emails arent makint it through the server maze either. All very annoying and unreliable.