The vacation program is not built specifically for Postfix.  So you will need to set it up separately from Postfix and then integrate it. The vacation feature allows you to configure a message that will be returned to the sender when you are out of the office.  If you are using a CentOS version you will not find vacation in any repositories so you will have to compile it, which is not too difficult.

# yum install gdbm-devel
# cd /usr/local/src/
# wget http://internap.dl.sourceforge.net/sourceforge/vacation/vacation-1.2.7.0.tar.gz
# tar -xvzf vacation*
# cd vacation-1.2.7.0
By default it will want to place the man pages in a different localtion so it may be easier just too create the necessary folders and be done with it.
# mkdir /usr/man
# mkdir /usr/man/man1
# make
# make install
The result will be that the binary will be placed in /usr/bin/vacation.

For each user, they must be local users, not virtual users, create a “.vacation.msg” file that looks something like this:
Subject: Out of the Office

I am out of the office until December 14.  I will contact you as soon as I return.

This file must be placed in the user’s home directory.

You must also create a “.forward” file in the user’s home.  The format should look like this:

\myuser, “|/usr/bin/vacation  myuser”

Be sure to replace “myuser” with the real username.

That should make it all work so that when you send a message to the user who is out of the office, Postfix will  return the message in the vacation file.