ErrUnhandled is used when a response hasn't been handled.

An AUTHENTICATE response.

A CAPABILITY response.

An ENABLED response, defined in RFC 5161 section 3.2.

An EXPUNGE response.

A FETCH response.

An IDLE response.

A LIST response.

A SEARCH response.

A SELECT response.

A STATUS response.

Handler handles responses.

Replier is a Handler that needs to send raw data (for instance AUTHENTICATE).

HandlerFunc is a function that handles responses.