tau

tau is a mail client that can hold multiple accounts in one window. it accepts imap and smtp to any host, so any mailbox that supports them should work.

mail stays local, except for the trip to your own servers. there is no telemetry and no update check. passwords go to the system keychain, connections use tls only, blocks tracking pixels/opened mail tracking pixels & [message] html is shown in a sealed box. search is local.