Absolutely love Nine - it works marvellously with my own IMAP and SMTP servers, my work O365 etc. And everything in the UI is configurable. I think Nine is actually the only Android app I've ever actually paid for - highly recommended it.
(*My only real complaint about Nine is if I try to import more than a few months worth of email history into the local cache it slows to a crawl and starts breaking. I'd pay for an app that locally indexes a few tens of GB of email history and makes it instantly searchable.)
It did take them (the new owners) awhile to get the O365 MFA support coded in, which is when I purchased Nine and used it for a year or so. Once AquaMail got their code updated to handle that, it was like a breath of fresh air coming back to the client. (the export/import of your settings - which are plentiful - works great, I've also upgraded phones many times)