Website: k9mail.app

K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email.

I’ve choosen this application a long time ago after a long research and I’m fond of it. It supports multiple accounts, has a sophisticated system to control fetching mail and displaying folders and is quite flexible / customisable. It integrates well with OpenKeyChain to support reading and writing of encrypted email.