Okay, so I have found the solution.
Log in your droplet via ssh from Putty (or other ssh tool).
1a. ) install sendmail: sudo apt-get install sendmail
cd the following directory: /srv/redmine/config/
create an you file and called it configuration.yml: configuration.yml
nano -w configuration.yml
paste the following config:
save this file: CTRL + X + S + Y
cd the following directory: /srv/redmine/tmp
create an new blank file an named it: restart.txt
restart the application: touch /srv/redmine/tmp/restart.txt
and voila, you can send notification mail from redmine :)