It's an adapter application for ActivityPub, the standard that backs the "fediverse" and apps like Mastodon, PeerTube, etc, to email. ActivityPub/the fediverse

It's an adapter application for ActivityPub, the standard that backs the "fediverse" and apps like Mastodon, PeerTube, etc, to email. ActivityPub/the fediverse is already designed very similar to how email works so a lot of the terminology intentionally overlaps: ActivityPub at the raw level has a concept of an inbox and an outbox on the server. Most clients to ActivityPub do a lot of stuff "in the middle", taking data from the Inbox and feeding it into complex (usually Postgres) databases and then present it all with mostly bespoke patterns: as Twitter-like feed (in Mastodon's case) or a YouTube-like site (in PeerTube's case). This client intentionally sticks to the raw "email-like" flow and applies it directly to "ancient" email technologies: using SMTP to send outgoing messages to the ActityPub servers in the "fediverse" and dumping things from incoming messages into a classic style Maildir email directory rather than a database of any other sort (so you can read the "fediverse" in just about any traditional email client as if it was email).