CentOS 7

Set Virtual Domain
Configure Postfix to use Virtulal Domain to send an email with another domain name different from original domain.
For example, present domain name ⇒ srv.world
new domain name ⇒ virtual.host
a User [cent] has an email address [cent@mail.srv.world],
a User [redhat] has an email address [cent@mail.virtual.host],
the user [redhat] uses the same name for before [@] with [cent].
[1] Set Virtual Domain.
[root@mail ~]#
vi /etc/postfix/main.cf
# add to the end of the file

virtual_alias_domains = virtual.host
virtual_alias_maps = hash:/etc/postfix/virtual
[root@mail ~]#
vi /etc/postfix/virtual
# add to the head of the file

cent@mail.virtual.host redhat
[root@mail ~]#
postmap /etc/postfix/virtual

[root@mail ~]#
systemctl reload postfix

Set new account to Email client and make sure to be able to send emails.