Assuming email server (for example, sendmail) is installed.
Step 1: add an entry to /etc/mail/virtusertable like:
user@domain.com user
Step 2: create a normal user in the system.
# sudo useradd user -s /sbin/nologin
(It is not necessary to have a home directory for this user!!!).
Step 3: specify the password for the user:
# sudo passwd user
