Please implement SRV DNS-record lookup (see here: https://wiki.xmpp.org/web/SRV_Records)

This way it is possible to provide an IP/port combination different to the A-record of the xmpp domain.
Even multiple SRV records with different priority are possible and should be tried in order by clients.

This allows server operators to even use port 443 (https) as fallback for environments where only http/https is allowed and all other ports are filtered out.

To multiplex https and xmpp (and possibly others) on port 443 a correct ALPN value of xmpp-client and support of _xmpps-client SRV records is needed.

One service provider (apart from myself) using port 443 multiplexing based on ALPN values and SRV records is conversations.im