If you get this error while running the iRedMail install script:
[FAIL] Starting nginx: nginx failed! invoke-rc.d: initscript nginx, action "start" failed. dpkg: error processing package nginx-full (--configure): installed nginx-full package post-installation script subprocess returned error exit status 1 Errors were encountered while processing: nginx-full E: Sub-process /usr/bin/dpkg returned an error code (1) > Installation failed, please check the terminal output. > If you're not sure what the problem is, try to get help in iRedMail > forum: https://forum.iredmail.org/
It's because nginx is arguing with Apache over ports. You need to stop Apache and start the script again, like this:
you@mx:~/iRedMail-1.3.1# sudo service apache2 stop
you@mx:~/iRedMail-1.3.1# sudo bash iRedMail.sh
That's all.