아직 해보진 않았는데 이런 글을 발견 했네요.
Try the following (it worked for me: I had a working flawless Xmail server).
After that the installation failed, open the file: /var/lib/dpkg/info/xmail.postinst
 you need to find this lines:
  if [ -z "$2" ]; then
 #we are installing for the first time
        cp /usr/share/doc/xmail/sample/*.tab /etc/xmail
        cp /usr/share/doc/xmail/sample/dnsroots /etc/xmail
        #make links to xmail config files
        ln -s /etc/xmail/*.tab /var/lib/xmail/
        ln -s /etc/xmail/dnsroots /var/lib/xmail
        chmod 711 /var/lib/xmail
you need to edit 4 lines starting with "cp ..." and "ln ..." as follow:
  if [ -z "$2" ]; then
 #we are installing for the first time
        cp -u /usr/share/doc/xmail/sample/*.tab /etc/xmail
        cp -u /usr/share/doc/xmail/sample/dnsroots /etc/xmail
        #make links to xmail config files
        ln -s -f /etc/xmail/*.tab /var/lib/xmail/
        ln -s -f /etc/xmail/dnsroots /var/lib/xmail
        chmod 711 /var/lib/xmail
In order to the install process the new xmail.postinst, I installed (and removed) a dummy application
NOTICE:
a minor bug is in the /var/lib/xmail/message.id file (it must contains
a pure number, so you need to change a line in the file /etc/init.d/xmail):
it must be changed BEFORE xmail starts.
find this line:
 echo -e "1\r\n\0" > /var/lib/xmail/message.id
and change it in:
 echo "1" > /var/lib/xmail/message.id
alternatively you may change manually the file /var/lib/xmail/message.id writing a unique line containing the number 1

출처 : https://bugs.launchpad.net/ubuntu/+source/xmail/+bug/128353      Marco Riccardi님의 글

월요일에 해봐야 겠네요..