Solution:
Modify the main.cf file including the line
rbl_reply_maps = hash:/opt/zimbra/postfix-2.6.5.2z/conf/rbl_reply_maps
Now create this file with the message to show, in my case the message isn "550 blocked by DNSBL.REDIRIS.ES"
$ more rbl_reply_maps
strong.dnsbl.rediris.es 550 blocked by DNSBL.REDIRIS.ES
Now you have to create the db file (rbl_reply_maps.db)
#postmap rbl_reply_maps
Just stop and start your mta
#zmmtactl stop
#zmmtactl start |