Implement the postfix part of domain aliasing

postfix/conf/sqlite-virtual_alias_maps.cf

@@ -4,7 +4,9 @@ query =
    FROM
      (SELECT destination, email, wildcard, localpart FROM alias
       UNION
-      SELECT (CASE WHEN forward_enabled=1 THEN (CASE WHEN forward_keep=1 THEN email||',' ELSE '' END)||forward_destination ELSE email END) AS destination, email, 0 as wildcard, localpart FROM user)
+      SELECT (CASE WHEN forward_enabled=1 THEN (CASE WHEN forward_keep=1 THEN email||',' ELSE '' END)||forward_destination ELSE email END) AS destination, email, 0 as wildcard, localpart FROM user
+      UNION
+      SELECT '@'||domain_name as destination, '@'||name as email, 0 as wildcard, '' as localpart FROM alternative)
    WHERE
      (
       wildcard = 0

postfix/conf/sqlite-virtual_mailbox_domains.cf

@@ -1,2 +1,5 @@
 dbpath = /data/main.db
-query = SELECT name FROM domain WHERE name='%s'
+query =
+  SELECT name FROM domain WHERE name='%s'
+  UNION
+  SELECT name FROM alternative WHERE name='%s'