I was thinking if I could get liblz4-tool properly installed first, the iRedMail install would work OK.
liblz4-1 is installed already (as v0.0~r131-2ubuntu2) so I tried this:

$ sudo apt remove liblz4-1
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  appstream gnome-software-common libappstream-glib8 libappstream3 libfwupd1 libgcab-1.0-0 libgtkspell3-3-0
  libpango1.0-0 libsnapd-glib1 python3-problem-report python3-xkit snap-confine snapd-login-service
  ubuntu-core-launcher
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  apport apport-gtk apt apt-transport-https apt-utils aptdaemon command-not-found flashplugin-installer gdebi
  gdebi-core gnome-software language-selector-common language-selector-gnome libapt-inst2.0 libapt-pkg5.0 libept1.5.0
  liblz4-1 lubuntu-core lubuntu-desktop lubuntu-software-center python-apt python-aptdaemon
  python-aptdaemon.gtk3widgets python3-apport python3-apt python3-aptdaemon python3-aptdaemon.gtk3widgets
  python3-aptdaemon.pkcompat python3-commandnotfound python3-distupgrade python3-software-properties
  python3-update-manager snapd software-properties-common software-properties-gtk squashfs-tools synaptic
  system-config-printer-gnome ubuntu-drivers-common ubuntu-minimal ubuntu-release-upgrader-core
  ubuntu-release-upgrader-gtk ubuntu-software ubuntu-standard ubuntu-system-service update-manager update-manager-core
  update-notifier update-notifier-common
WARNING: The following essential packages will be removed
This should NOT be done unless you know exactly what you are doing!
  apt libapt-pkg5.0 (due to apt)
0 to upgrade, 0 to newly install, 49 to remove and 0 not to upgrade.
After this operation, 78.7 MB disk space will be freed.
You are about to do something potentially harmful.
To continue type in the phrase 'Yes, do as I say!'
?] n
Abort.
$

So that looks like liblz4-1 is part of the pseudo-package lubuntu-desktop, and removing it removes lubuntu-desktop.
But liblz4-tool is not part of that pseudo-package, and won't install because of the name problem.  But it looks like that is due to the way liblz4-tool is packaged - it should be asking for  liblz4-1 (= 0.0~r131-2ubuntu2) but it is in fact asking for  liblz4-1 (= 0.0~r131-2).

http://packages.ubuntu.com/xenial/liblz4-tool
Package: liblz4-tool (0.0~r131-2ubuntu2) [universe]
Dependencies:
    liblz4-1 (= 0.0~r131-2ubuntu2)

Have I got that right?

Thanks, but exactly the same.

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$ sudo apt update
[sudo] password for ***:           
Hit:1 http://ubuntu.01link.hk xenial InRelease                                                                         
Hit:2 http://ppa.launchpad.net/ubuntugis/ubun … ble/ubuntu xenial InRelease                                 
Ign:3 http://dl.google.com/linux/earth/deb stable InRelease                                   
Hit:4 http://ubuntu.01link.hk xenial-updates InRelease             
Hit:5 http://dl.google.com/linux/earth/deb stable Release                                                             
Hit:7 http://ubuntu.01link.hk xenial-backports InRelease                 
Hit:8 http://ubuntu.01link.hk xenial-security InRelease                                             
Hit:9 http://qgis.org/ubuntugis xenial InRelease   
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.
$ sudo apt upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.
$ cd /path/to/iRedMail-0.9.6
$ sudo bash iRedMail.sh
awk: cannot open /etc/ssh/sshd_config (No such file or directory)
[ INFO ] Checking configuration file: /***/iRedMail-0.9.6/config ... [FOUND]
< Question > Use it for mail server setting? [y|N]y
[ INFO ] Use config file: /***/iRedMail-0.9.6/config for mail server setting.
[ INFO ] Import installation process status from file: /***/iRedMail-0.9.6/runtime/install.status.
[ INFO ] Installing package(s): postfix postfix-pcre mariadb-client mariadb-server postfix-mysql libdbd-mysql-perl dovecot-imapd dovecot-pop3d dovecot-lmtpd dovecot-managesieved dovecot-sieve dovecot-mysql amavisd-new libcrypt-openssl-rsa-perl libmail-dkim-perl clamav-freshclam clamav-daemon spamassassin altermime arj zoo nomarch cpio lzop cabextract p7zip rpm ripole libmail-spf-perl unrar-free pax python-sqlalchemy python-dnspython python-mysqldb python-pymysql python-jinja2 python-netifaces python-webpy python-beautifulsoup python-lxml python-pycurl python-bcrypt fail2ban bzip2 acl patch cron tofrodos logwatch unzip bsdutils liblz4-tool
Reading package lists...
Building dependency tree...
Reading state information...
acl is already the newest version (2.2.52-3).
acl set to manually installed.
bzip2 is already the newest version (1.0.6-8).
cpio is already the newest version (2.11+dfsg-5ubuntu1).
patch is already the newest version (2.7.5-1).
pax is already the newest version (1:20151013-1).
pax set to manually installed.
python-jinja2 is already the newest version (2.8-1).
python-jinja2 set to manually installed.
python-lxml is already the newest version (3.5.0-1build1).
python-lxml set to manually installed.
python-netifaces is already the newest version (0.10.4-0.1build2).
python-netifaces set to manually installed.
rpm is already the newest version (4.12.0.1+dfsg1-3build3).
rpm set to manually installed.
unzip is already the newest version (6.0-20ubuntu1).
unzip set to manually installed.
bsdutils is already the newest version (1:2.27.1-6ubuntu3.1).
cron is already the newest version (3.0pl1-128ubuntu2).
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
liblz4-tool : Depends: liblz4-1 (= 0.0~r131-2) but 0.0~r131-2ubuntu2 is to be installed
E: Unable to correct problems, you have held broken packages.
<< ERROR >> Installation failed, please check the terminal output.
<< ERROR >> If you're not sure what the problem is, try to get help in iRedMail
<< ERROR >> forum: http://www.iredmail.org/forum/
$
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