When you run the command newaliases, it will:

A.
ask for input on stdin to create new mail aliases.

B.
restart sendmail.

C.
remove the aliases currently configured.

D.
rebuild the aliases database for the file /etc/aliases.

Explanation:
From the man pages:
newaliases rebuilds the random access data base for the mail aliases file /etc/aliases. It must be run each time this file is changed in order for the change to take effect.