kumomta/docs/reference/addressheader/user.md

addressheader.user

If the address header consists of a single simple address, returns the mailbox portion of the address. For example, if the address is "first.last@example.com", addressheader.domain will evaluate as "first.last".

If the address header is not a single simple address, raises an error.

See also addressheader.domain, addressheader.name.