go to preferences, sharing and change the name of both rendezvous name and computername.
in x2 there is probably a bug with rendezvous that affects the integrated mail-client of jaguar in combination with mail-servers that are protected with spam-filters. (you can not send mails via smtp, but you can receive without problems)
The best way to change the computername is to log in as root. go to folder /etc and change the hostname in the file hostconfig. (replace "-AUTOMATIC-" with your valid hostname like this "myhost.local"