I think kiltbear might be on to something.
Some Jabber clients allow the user to specify a different "connect" server than the hostname which is a part of your username. For example, GTalk accounts are @gmail.com, but they actually connect to talk.google.com.
For the Google Hosted email account
xx@x3.com, it's possible that Meebo is trying to connect to a Jabber server on x3.com when it should try to connect to talk.google.com.
I guess we would need to add a "Connect Server" option. Or we could separate the Jabber and GTalk login boxes. Or we could check to see if the DNS for x3.com has an SRV record that points to the Jabber server on talk.google.com, but then all Google Hosted domains would need to configure an SRV record.