Ease of use and the ability to merge all emails in one place

K9mail handles multiple accounts easily, notifications are trivial to set up, and of course the entire app is open source. It is updated fairly frequently- every month or two, usually. There are no major features which are absent, and it can be used with any mail server which supports IMAP or POP. It has built-in support for PGP encryption as well, using an app like OpenKeychain.

Free, supports multiple inboxes and IMAP and POP3