The s2s-skip-tls-hostnames property disables TLS handshaking for s2s connections to selected remote domains. Unfortunately some servers (certain versions of Openfire - [1] or [2]) have problems with TLS handshaking over s2s which prevents establishing a usable connection. This completely blocks any communication to these servers. As a workaround you can disable TLS for these domains to get communication back. Enabling this can be done on any vhost, but must be configured under the s2s component.

s2s {
    'skip-tls-hostnames' = [ 'domain1', 'domain2' ]