ejabberd/EXTRAS/example-configurations/conf.d/41-s2s.yml

# starttls is gloablly required for s2s 
max_stanza_size: 131072

 -
    port: 5269
    ip: "::"
    module: ejabberd_s2s_in
    starttls_required: true
    max_stanza_size: 524288
  -
    port: 5270
    ip: "::"
    module: ejabberd_s2s_in
    tls: true
    max_stanza_size: 524288