Hi Marco,
When Exim is installed on Ubuntu, its configurations can be found under /etc/exim4/

On Ubuntu, Exim configurations can be split into multiple files which are kept under /etc/exim4/main/ or present in a single file /etc/exim4/exim.conf.template

Once you make any changes to configuration,  to get those into effect you will need to run below commands:

1. update-exim4.conf

2. service exim4 restart

For more information related to Exim on Ubuntu, you can also consider visiting https://wiki.debian.org/Exim


However, if you are using CentOS, then Exim configurations can be found under /etc/exim/exim.conf file.
Exim reads configuration information from this file when it is started.
To bring into effect any configuration changes made, you also need to restart Exim Service by running below command:

service exim restart