As far as I can tell, /etc/mailname is created by the postfix postinstall script.

root@foobar ~ # debconf-get-selections | grep mailname
postfix	postfix/mailname	string	grml.mycurrentdomain.tld

I'm not sure whether the mail name should be fixed by preseeding or if it should be set 
in the hostname function in chroot-script, too.