Well, in case of mutt, it's just reading email from a local maildir. That is, a MUA (mail user agent) in it's own jargon.
Mostly everything else is outsourced. Composing is done by your $EDITOR, sending is done by an external program, fetching email by an external program too. Even indexing and querying is usually outsourced to a modern xapian-based tool, like mu or notmuch.
Mostly everything else is outsourced. Composing is done by your $EDITOR, sending is done by an external program, fetching email by an external program too. Even indexing and querying is usually outsourced to a modern xapian-based tool, like mu or notmuch.
This leads to powerful composable workflows.