[opensmtpd]

I've also been satisfied with Wormhole[1]. Been using it ever since Firefox Send was discontinued.

  > If you like Instant.io, try Wormhole from the creator of WebTorrent and Instant.io. It's like Instant.io, but adds end-to-end encryption and links that keep working even after you close your browser.

[1] https://wormhole.app

[SubiculumCode]

I loved Firefox Send, but part of its appeal was I know Mozilla as an organization.