AVOID THIS CRAP. Only reason I bought it is because at that time I wanted to use Mail 'cause I didn't see any benefit on running a different mail client given Mail is quite a good app -- so investing in a plugin instead buying another app seemed the correct choice. And I still believe it is, as I still use and like Mail... the problem is with this very specific piece of crap that MailButler is.
From the same people that brought you Shimo, the OpenVPN client that will let you stranded more than often by screwing your .ovpn configuration files stored on the cloud (it's 2019 and they don't have it right yet), or will lose connection and force you to jump into the terminal in order to manually kill the OpenVPN process to regain access to internet -- please give a warm reception to MailButler, the Mail app plugin that shoves down your throat the way it's supposed to use it!
This MB crap is one of the most invasive apps I met since moving into Mac about four years ago.
I bought the Professional plan a year ago or so since the price was correct for a lifetime license. I'd like to mention that at that time MB was still usable, giving the user full ownership over it's behavior.
Fast-forward some months, they "evolved" it to version 2 and then was when everything started to go downhills: first of all, they removed the configuration pane from the plugin installed on your local system to their cloud-based service while removing a wealth of features in the process; secondly, they never provided a way to disable MB from adding itself a menu bar icon that no matter how many times you quit it, it will repeatedly spawn over and over again without asking the user if that's what he wants; thirdly, this software constantly installs itself as login item (again, no matter how many times you remove the launch daemon, it will magically be there again next time you check the list).
Oh, and I haven't mentioned yet their rude, condescending support for both MB and Shimo. To be honest, looks like they don't give a single F about their users, to them we are just cows to milk and treat like sheit.
If you're really looking for the functionality marketed by MB, I strongly suggest you go and check the alternatives out there, like the Boomerang plugin for Gmail (among others).