Who consumes what lands on the channel: the closed recipient-class set
(§7.1.5, RFC 0005).
The SECOND DIMENSION the a2a_email request was really asking for: a
refinement that must be applied to every member of a set is not a member
of that set, so “agent mail” is kind: email + recipientClass: agent
rather than an eighth kind that doubles the vocabulary. Two values,
closed at two DELIBERATELY — grown by amendment like every closed set
here, not by a producer minting one.
A recipient class asserted by the sender is a CLAIM, not a proof: it
constrains an honest-but-over-broad agent — the actual threat, one’s own
agent doing more than one meant — and does not constrain a hostile one.
No caller should present it as the latter.
The inverse of RecipientClass::label. Error shape per
[ChannelKind::from_str]’s documentation: a plain message, because
§8.3 step 1 classifies an unrecognized value as a strict-parse
rejection, below the protocol’s closed set of error codes.