This configuration is made to respond to each incoming email by an automated message.
It is inspired from the site:
http://www.akadia.com/services/automatic_email_answering.html

In Debian: Install the package vacation
apt-get install vacation

Assumptions:
System User: marie
Email address: marie.larue@myserver.com

Configuration:

Initialize the database file ~/.vacation.db
sudo -u marie vacation -I

Create the following 2 files in the home directory of the user marie
.forward
.vacation.msg

Content of .forward
\marie, "|/usr/bin/vacation -a marie.larue -r 0 marie"

Content of .vacation.msg
From:marie.larue@myserver.com
Subject: Out-of-office.
.
Dear client,
I will be on vacation till .....
Please do not respond to this automated email.
.
Best regards
Marie Larue

Please ensure that both files .vacation.msg and .vacation.db are owned by the respective user(marie), if not, you will have permission problems and this will not work as expected.
After these 2 files will be created all incoming emails will be as usual be added to Marie’s mailbox but will also automatically be responded to the sender with the above message.

If for some reason this is not working as expected you can check the mail logs to see what is wrong.
tail -f /var/log/mail.log