There is a mailing list (of course!) called "xcmail" and a announcement list called "xcmail-announce". Users of the list "xcmail" are automatically part of "xcmail-announce" but not vice versa!
To subscribe send an email to email@example.com with a text body of subscribe xcmail - or use XCmail: look the right pull down menu "Xclasses", there in "mailing lists/Subscribe .....". This will open a newmail window with all addresses and textes ready - simple press "Send".
To write to the list send a mail to "firstname.lastname@example.org" or use this nice XCmail menu ("Xclasses/mailing list/Write to").
To unsubscribe the list send an email to email@example.com with a text body of unsubscribe xcmail.
But before you are going to ask something you should better have a look in the XCmail FAQ ;-)
All written mails of the mailing list can be read from a nice web frontend.