Hello @sahsanu sir,
If you remember I face the same problem few months ago
After panel update, mail can’t connect to third-party email apps
I mentioned it again in here because that time I also use bluemail app and it work that time also same like @luchtbakker but not the other Android app like outlook and Gmail. I noticed that it only happen in Android outlook app not in desktop/pc. I think it will stay like this but when new update given in playstore of Gmail and outlook it started work again, I am able to add my mail account (generated in Hestia) in this Android app again.
I don’t know am i right or not, but I think this problem arise because of some Android OS problem not because of hestiacp. Because it started working again after update given by playstore, also my mail work fine in PC in all other mail client fine during that time.
I suggest @luchtbakker sir if possible try again to add account in your Android mail app again after new update given in play store (app store in case of apple).