126 Chapter 5: Jabber Technology Basics routed. Different email users have different home mailboxes. In the same way, the JIDs of different Jabber users reflect each user's home Jabber server, to which everything addressed to his JID is routed. A message addressed by a user based on one Jabber server to a user based on another Jabber server is automatically routed from the one server to the other. Rules and Regulations A JID must contain a hostname part to be valid. The username and resource parts are optional; circumstance and usage dictates when either of these parts is neces- sary. A username is specific to the hostname that it's paired up with. For example: qmacro@jabber.org is not the same as qmacro@jabber.com . There are some restrictions on how each JID part is composed; Table 5-1 details these restrictions. Although you can be particular about the case of letters in a username, any operations (such as comparisons) at the Jabber server are case- insensitive. For example, if a user has registered dj as his username, then another user cannot register with the username DJ . However, the person who registered as dj can connect and send DJ when he authenticates, and for the duration of that session will be known as DJ not dj .