RFC 6121 requires a
<service-unavailable/> stanza error to be sent to a user messaging an unavailable recipient if the message is not stored for delayed delivery (i.e. as an "offline message").
If the recipient exists (i.e. auth module returns
mod_mam stores the message, but
mod_offline_stub is not tightly coupled with
mod_mam. It can be used as a standalone extension, if the specific application requires it.