When looking at https://igniterealtime.atlassian.net/browse/OF-2321, I suspect that there is a chance that stanzas destined for a to-be-established S2S domain can be delivered out of order.

While the outgoing session is being set up, stanzas are being queued. This queue is flushed after the session has been established.

Stanzas that are sent after the outgoing session has finished being set up will be delivered immediately. There is no guarantee that at this time any queued stanzas have already been sent.