The best way to ensure you have a copy of all your emails is to use an email client/app that is set to download your email folders and contents onto your device and you can then include them into you backup regime, assuming you have one.
By doing that you become responsible for ensuring the safe storage of your emails rather than depending on a third party to do it.
As regards the actual deletion do you access your email via a webrowser and do you use more than one device?
Snap !! Can we get together on this … I’ve lost all emails from 2017 to this morning. I still have sent . I haven’t deleted or sent to another folder . I’m not on POP . I’ve checked Blockers and Rules …. They have just disappeared….. I’m going on holiday at 4am tomorrow and all my parking , flight, hire car , insurances and campsite details have gone missing. I’m missing medical info , work contract bookings for next three months and a whole host of other important information, some of which I won’t realise until I need it !!! I note this has been an issue over the years … spent over an hour with chat function … useless !!! Arggggghhh