Establishing S2S can take some time. Stanzas that are sent to the remote domain while this is in progress, are being queued.

The size of that queue should be larger than the current default (50). We've observed that people from a remote domain joining a MUC room (via an inbound connection), causes an outbound connection (promise) to be created, over which MUC history is sent. This can very quickly fill up the queue, leading to lost data.