I very much need a feature to group Mail Accounts like e.g. in the Postbox or Thunderbird apps. I have more than a dozen accounts, some of which are for specific purposes.

So I like a group of "priority" accounts.
another group of "private" accounts,
yet another group of "-Commerce" accounts,
plus a group of "mailing list" accounts etc...

Each with separate unified inbox, customizable notifications, the option to quickly switch per group, a GUI feature to collapse / expand all contained accounts etc.

And the top feature would also be to have accounts be part of multiple groups (even PostBox/Thunderbird do not have this). So an E-Mail account can both be "priority" and "private", or "e-commerce" and "mailing-list"....