Redmine email notifications. Documentation vs Reality
You want setup email notifications for Redmine which run's under Debian Jessie. So, you going Aministration-Settings-Email notifications and see notification which says that you need go and edit /etc/redmine/<instance>/configuration.yml file. And you going into /etc/redmine and looking for your instance (default in most cases) and you can't find any configuration.yml
Let's fix it!
You can find file configuration.yml.example in the /usr/share/redmine/config/ directory (surprise!). According to documentation, you need copy and edit this file.
$ sudo cp /usr/share/redmine/config/configuration.yml.example /usr/share/redmine/config/configuration.yml
Next you need setup your email account, according to documentation - EmailConfiguration
Now let's make this configuration visible for your Redmine.
$ sudo ln -s /usr/share/redmine/config/configuration.yml /etc/redmine/default/configuration.yml
Let's restart Thin (in my case) and Nginx and check Aministration-Settings-Email notifications widget
Hope it will help you.