The connection between your client and xmpp.net is encrypted using TLS. The server prefers secure connections to other servers, however this cannot be guaranteed.
Since you can't (and shouldn't) trust your xmpp server, I recommend you always enable [Off-The-Record (OTR)][off-the-record] in your chat client. This will give you end-to-end encrypted chat that cannot be decrypted while in transit
and, depending on the settings of your client, will not be logged on your local machine.
xmpp.dk allows federated connections to all other S2S TLS-enabled Jabber servers so you can chat with anyone that has a Jabber account. Note that Google Talk (has this been deprecated yet?) doesn’t support S2S connections.