Open the aliases in the text editor:
sudo nano /etc/aliases
Example of redirecting the root mail to admin@example.net:
root: admin@example.net
You can forward directly to several addresses:
root: admin@example.net,user@example.net
Press Ctrl+O then Enter and Ctrl+X to exit the nano editor.
For the changes to take effect, you must update the alias database with the command:
sudo newaliases
For exim with cPanel, update this with:
exim -bi
If postfix is used as the mail sending server, then the /etc/postfix/main.cf config file can specify other paths to the list and the alias base, for example:
alias_maps = hash:/etc/postfix/aliases
alias_database = hash:/etc/postfix/aliases
In this case, edit this file too:
sudo nano /etc/postfix/aliases
cd /etc/postfix/aliases
sudo newaliases
sudo service postfix restart
For verification, you can send an email to the user as root:
echo 'Test' | mail -s Test root
Se also my article:
Solution of the warning “database /etc/aliases.db is older than source file /etc/aliases”