More details:

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root@xenial:/usr/local/src/imscp# LANG=C aptitude install courier-authdaemon
The following NEW packages will be installed:
  courier-authdaemon 
0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/5756 B of archives. After unpacking 39.9 kB will be used.
Selecting previously unselected package courier-authdaemon.
(Reading database ... 101286 files and directories currently installed.)
Preparing to unpack .../courier-authdaemon_0.66.4-2_amd64.deb ...
Unpacking courier-authdaemon (0.66.4-2) ...
Processing triggers for systemd (228-5ubuntu3) ...
Processing triggers for ureadahead (0.100.0-19) ...
Setting up courier-authdaemon (0.66.4-2) ...
[/usr/lib/tmpfiles.d/courier-authdaemon.conf:2] Unknown user 'courier'.
Job for courier-authdaemon.service failed because the control process exited with error code. See "systemctl status courier-authdaemon.service" and "journalctl -xe" for details.
invoke-rc.d: initscript courier-authdaemon, action "start" failed.
dpkg: error processing package courier-authdaemon (--configure):
 subprocess installed post-installation script returned error exit status 1
Processing triggers for systemd (228-5ubuntu3) ...
Processing triggers for ureadahead (0.100.0-19) ...
Errors were encountered while processing:
 courier-authdaemon
E: Sub-process /usr/bin/dpkg returned an error code (1)
Failed to perform requested operation on package.  Trying to recover:
Setting up courier-authdaemon (0.66.4-2) ...
[/usr/lib/tmpfiles.d/courier-authdaemon.conf:2] Unknown user 'courier'.
Job for courier-authdaemon.service failed because the control process exited with error code. See "systemctl status courier-authdaemon.service" and "journalctl -xe" for details.
invoke-rc.d: initscript courier-authdaemon, action "start" failed.
dpkg: error processing package courier-authdaemon (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 courier-authdaemon
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As you can see, installation failed. This is due to the wrong user/group put in the /usr/lib/tmpfiles.d/courier-authdaemon.conf:

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root@xenial:/usr/local/src/imscp# cat /usr/lib/tmpfiles.d/courier-authdaemon.conf
#Type Path                        Mode UID     GID     Age Argument
    d /var/run/courier/authdaemon 0755 courier courier -   -
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Changing it to daemon user/group solve the problem:

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root@xenial:/usr/local/src/imscp# nano /usr/lib/tmpfiles.d/courier-authdaemon.conf
root@xenial:/usr/local/src/imscp# cat /usr/lib/tmpfiles.d/courier-authdaemon.conf
#Type Path                        Mode UID     GID     Age Argument
    d /var/run/courier/authdaemon 0755 daemon daemon -   -
root@xenial:/usr/local/src/imscp# apt-get install -f
Lecture des listes de paquets... Fait
Construction de l'arbre des dépendances       
Lecture des informations d'état... Fait
0 mis à jour, 0 nouvellement installés, 0 à enlever et 0 non mis à jour.
1 partiellement installés ou enlevés.
Après cette opération, 0 o d'espace disque supplémentaires seront utilisés.
Paramétrage de courier-authdaemon (0.66.4-2) ...
root@xenial:/usr/local/src/imscp#
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