Domain name "test.de" is pointed via DNS MX entry to the MTA Server "antispam.mxserver.de" (anti spam server).
The anti spam server is just a transport server, to check the mail for spam and forward it to the REAL MX Server
which is "mx1.mxserver.de". For this procedure, postfix needs 4 settings: "virtual_alias_maps", "relay_recipient_maps",
"transport_maps" and "relay_domains". In the right now version it is done my mysql:. As for this, its needed to
do 3 mysql queries to: domain_relay, virtual_user (catchall), transport (destination mx) per domain name.
The new solution should be over DNS Query Requests. A domain name has 2 entries. One standard MX Entry with the
antispam server. And an additional TXT entry with an encrypted string which contains the destination mx (transport).
Thats all. I the encrypted string is existing, postfix accepts the domain (relay_domains) and takes all receptions
(virtual_alias_maps) and knows from the encrypted string the destination mx (transport_maps).
The closer details (about encrypted string, and so on) will be discussed on start. Basically it must be a very fast addon, not much to do, just to understand the postfix source code.