Connection vanished
The connection was broken before the exchange finished.
In detail
The uncomfortable outcome: if it happens after the content was sent, whether the message arrived is genuinely unknown. The engine's behaviour is configurable through the flags, and whichever way it is set, one of duplicate delivery or silent loss is possible. For analytics it should be visible rather than folded into failures, because a rise in lost against one receiver often means a middlebox, not a policy.