Currently we use the list from xmpp.org. To be in that list you have to send an email to the Operators list. Unafortunately, xmpp.org's list hasn't updated since almost a year. I have been talking with one of the maintainers, and the list is at the top of his TODO list, but if it doesn't get up to date shortly, probably I will add manually the servers wich sent emails to the Operators list.