> and I never noticed that it's got a bad interface
That's because you are just accustomed to it [0]. If you would have a fresh look on it, especially on a small screen - it would find it atrocious.
There is still a separate SMTP configuration and it's a separate separate SMTP configuration for the each account.
There is absolutely useless left vertical bar which... does nothing what can't be shoved unto the useless, empty top bar. There is useless empty top bar which holds a search box and a hamburger menu.
For the unknown reason[1] this top bar is twice the tall of the regular menu. Oh. There is TWO search boxes[2]. Enabling Quick Filter adds another toolbar instead of using the bar where the button 'Quick Filter' resides.
And if you have multiple accounts and for some reason you can't auth for a moment (eg server problem, not with a connection) you would be spammed with the auth boxes and even if you successfully auth in one - you would still need to cancel or auth in all other what was spammed on you.
There is a lot, a lot of things where TB sucks. You just familiar with it.
[0] and probably configured it years ago and never touched the config too.
[1] actually it to be tall enough to hold hamburger button - because there is an option to add buttons to it. But you can't move buttons from the left useless vertical bar to the top bar. So you can have 'Mail', 'Calendar' etc duplicated on the top bar... for what?
NB yes you can hide the left vertical bar. Hide, not remove. Still it was thought on, implemented and shoved on our asses for absolutely no reason..
[2] https://news.ycombinator.com/item?id=44521073