MTA: postfix
vacation program: gnarwl
LDAP: openldap 2.2.13-4
Horde version: 3.1.3
vacation version: 3.0

(For those who don't want to use gnarwl, a "traditional" vacation program for ldap is available at http://www.etc.msys.ch/sources/ldapvacation/).

First, define a postfix transport map for your specific internal autoreply domain.
If your domain is "example.com" and your autoreply domain is to be "autoreply.example.com", then you would add the following to
/etc/postfix/transport:

autoreply.example.com    gnarwl:

Next, create the database files from the above file. The following command should create the appropriate transport.db file:

postmap hash:transport

Next add the transport to the postfix main.cf file in the "TRANSPORT MAPS" section:

# TRANSPORT MAPS
#
transport_maps = hash:/etc/postfix/transport

Now add a new service to the /etc/postfix/master.cf file:

# gnarwl autoresponder
gnarwl    unix  -       n       n       -       -       pipe
  flags=F  user=gnarwl argv=/usr/bin/gnarwl -a $user

Create an LDAP vacation alias map called ldap-virtual_alias_maps-vacation.cf:

server_host = ldap://ldap.example.com
version = 3
server_port = 389
search_base = ou=mail,...
query_filter = (&(objectClass=MailAccount)(objectClass=Vacation)(mail=%s)(accountActive=TRUE)(vacationActive=TRUE))
result_attribute = mail
result_filter = %s,%s@autoreply.example.com
bind = no

Of course, the above needs to reflect your ldap environment...

Append the alias map to LDAP's main.cf file (make sure it's the first map listed):

virtual_alias_maps =
ldap:/etc/postfix/ldap-virtual_alias_maps-vacation.cf,
ldap:/etc/postfix/ldap-virtual_alias_maps.cf

after those changes, reload postfix:

postfix reload

Extend your LDAP with the vacation schema and setup a test account. An example ldif follows:

dn:mail=vacationtest@example.com,...
objectClass: top
objectClass: Vacation
mail: vacationtest@example.com
accountActive: TRUE
[... other mail specific attributes ...]
vacationActive: TRUE
vacationInfo:: [vacation message has to be utf-8 encoded and also base64 if necessary]

After loading the above ldif, test your postfix alias vacation lookup map:

postmap -v -q vacationtest@example.com
ldap:/etc/postfix/ldap-virtual_alias_maps-vacation.cf

This should return the following if the vacation has been enabled, otherwise nothing.

vacationtest@example.com,vacationtest@example.com@autoreply.example.com

If this works so far, configure gnawrl to suit your needs (only the ldap part and the header config is included):

Edit gnarwl.cfg and change the following lines:

server ldap.example.com
protocol 3
base ou=mail...
queryfilter
(&(objectClass=MailAccount)(objectClass=Vacation)(mail=$recepient)(accountActive=TRUE)(vacationActive=TRUE))
mta /usr/sbin/sendmail $sender
charset
forceheader /var/lib/gnarwl/header-utf8.txt
loglevel 3

Create the file /var/lib/gnarwl/header-utf8.txt and add the content shown below:

From: $fullname <$recepient>
To: $sender
X-mailer: GNARWL
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Subject: Re: $subject

Finally you will have to configure the sork vacation app with the ldap driver. This should be an easy task now ;)
Remember, that you have to configure/specify the vacation related attributes.