diff --git a/documentation/database/patches/patch-0003-add-host-mda.sql b/documentation/database/patches/patch-0003-add-host-mda.sql
new file mode 100644
index 0000000..7642d19
--- /dev/null
+++ b/documentation/database/patches/patch-0003-add-host-mda.sql
@@ -0,0 +1,30 @@
+CREATE TABLE `{$prefix}host` (
+	`host_ID`          INT UNSIGNED NOT NULL AUTO_INCREMENT,
+	`host_ipv4`        INT UNSIGNED NOT NULL COMMENT 'main IPv4 stored with INET_ATON()',
+	`host_hostname`    VARCHAR(128) NOT NULL COMMENT 'hostname',
+	`host_description` TEXT,
+	PRIMARY KEY (`host_ID`)
+) ENGINE  = InnoDB
+  COMMENT = 'a generic host';
+
+INSERT INTO `{$prefix}host`
+       (`host_ID`, `host_ipv4`,            `host_hostname`, `host_description`)
+VALUES (NULL,      INET_ATON('127.0.0.1'), 'localhost',     'This is the default server.\r\n\r\nPlease set your absolute IP address and your hostname.');
+
+CREATE TABLE `{$prefix}mta` (
+	`mta_ID`  INT(10) unsigned NOT NULL AUTO_INCREMENT,
+	`host_ID` INT(10) unsigned NOT NULL,
+	PRIMARY KEY (`mta_ID`),
+	KEY `host_ID` (`host_ID`),
+	CONSTRAINT `mta_ibfk_1` FOREIGN KEY (`host_ID`) REFERENCES `{$prefix}host` (`host_ID`) ON DELETE CASCADE
+) ENGINE  = InnoDB
+  COMMENT = 'known mail delivery agents e.g. Postfix instances';
+
+INSERT INTO `{$prefix}mta`
+	( `mta_ID`, `host_ID` )
+VALUES  ( NULL,     1         );
+
+ALTER TABLE `{$prefix}domain`
+	ADD        `mta_ID` INT UNSIGNED NULL AFTER `domain_parent`,
+	ADD INDEX (`mta_ID`),
+	ADD CONSTRAINT `domain_ibfk_mta` FOREIGN KEY (`mta_ID`) REFERENCES `{$prefix}mta`(`mta_ID`) ON DELETE RESTRICT ON UPDATE RESTRICT;
diff --git a/documentation/etc/postfix/mysql/mysql-domains.cf b/documentation/etc/postfix/mysql/mysql-domains.cf
index c9715bf..0741f27 100644
--- a/documentation/etc/postfix/mysql/mysql-domains.cf
+++ b/documentation/etc/postfix/mysql/mysql-domains.cf
@@ -1,19 +1,25 @@
 #####################################################################
 # This Postfix configuration was provided from the KISS Hosting Panel
 #
 # Project homepage:
 #   https://gitpull.it/project/profile/15/
 #
 # Happy hacking!
 #
 # -- Valerio Bozzolan - Thu 10 Apr 2016 03:34:07 AM CEST
 #####################################################################
 
 #
 # This configuration file is read by 'virtual_mailbox_domains'.
 # Read 'main.cf' for further information.
+#
+# IMPORTANT: make sure that in your `mta` table you have at least one row
+#   and that its ID specified here is the same.
+#
+# Note: The mta_ID is useful if in the future you want to handle
+#   multiple mailservers with a single shared authentication database.
 user     = {$YOUR_POSTFIX_READONLY_MYSQL_USERNAME}
 password = {$YOUR_POSTFIX_READONLY_MYSQL_PASSWORD}
 dbname   = {$YOUR_DATABASE_NAME}
-query    = SELECT `domain_name` AS virtual FROM `{$YOUR_DATABASE_PREFIX}domain` WHERE `domain_name` = '%s' AND `domain_active` = 1
+query    = SELECT `domain_name` AS virtual FROM `{$YOUR_DATABASE_PREFIX}domain` WHERE `domain_name` = '%s' AND `domain_active` = 1 AND `mta_ID` = {$YOUR_DEFAULT_MTA_ID}
 hosts    = 127.0.0.1