Cannot install QMail: dpkg: error processing package postfix-pcre

Symptoms

• Cannot switch from Postfix to QMail with the following error:

  PLESK_INFO: E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution). ---X--- `apt-get` output ---------------------
  Reading package lists...
  Building dependency tree...
  Reading state information...
  You might want to run 'apt-get -f install' to correct these:
  The following packages have unmet dependencies:
  plesk-mail-pc-driver : Conflicts: psa-mail-driver
  Conflicts: psa-qmail but 3:1.03-debian8.0.build1700161026.19 is to be installed
  plesk-mail-qc-driver : Depends: psa-qmail-rblsmtpd but it is not going to be installed
  Conflicts: postfix but 2.11.3-1+deb8u2 is to be installed
  Conflicts: psa-mail-driver
  postfix : Conflicts: mail-transport-agent
  psa-qmail : Conflicts: mail-transport-agent
  ---------------------- `apt-get` output ---X---

• Execution of apt-get -f install fails with the error:

  PLESK_INFO: Removing postfix-pcre (2.11.3-1+deb8u2) ...
  /var/lib/dpkg/info/postfix-pcre.prerm: 18: .: Can't open /usr/share/postfix/postinst.functions
  dpkg: error processing package postfix-pcre (--remove):
  subprocess installed pre-removal script returned error exit status 2
  /var/lib/dpkg/info/postfix-pcre.postinst: 25: .: Can't open /usr/share/postfix/postinst.functions
  dpkg: error while cleaning up:
  subprocess installed post-installation script returned error exit status 2
  Errors were encountered while processing:
  postfix-pcre
  E: Sub-process /usr/bin/dpkg returned an error code (1)

• SpamAssasin component is present in the server:

  # plesk installer --select-release-current --show-components | grep spam
  ...
  Detecting installed product components.
  spamassassin [up2date] - SpamAssassin

Cause

This behavior is caused by a bug in the product with ID #PPPM-10498 which will be fixed in future Plesk updates

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Resolution

To fix it when this happens, the following can be performed:

1. Connect to the server via SSH

2. Execute the commands below:

   # mkdir -p /usr/share/postfix/ && echo 'delmap() { :; }' > /usr/share/postfix/postinst.functions
   # apt-get install -f
   # rm -rf /usr/share/postfix
   # /opt/psa/bootstrapper/pp17.8.11-bootstrapper/bootstrapper.sh post-install mail-qc-driver
   # /opt/psa/bootstrapper/pp17.8.11-bootstrapper/bootstrapper.sh perform-deferred-actions
   # rm /var/lock/parallels-panel-upgrade-failure.flag
   # plesk installer remove --components postfix
   # plesk installer update --repatch

3. If SpamAssassin component is still needed, install it with the following command:

   # plesk installer add --components spamassassin

Note: In order to avoid this problem when switching from Postfix to Qmail while SpamAssassin component is installed, the steps below should be performed:

1. Connect to the server via SSH

2. Execute the commands below:

   # plesk installer remove --components spamassassin
   # plesk installer add --components qmail
   # plesk installer add --components spamassassin