On plesk you can setup an external MySQL server.

With dns I have found a way of using external name servers as well. (I will write a tutorial soon about that) maybe someone will be helped from that. We have been using it for over a year and works solid !

With mail ... Is there anyway to make plesk use an external server and write on an external sql db that has records of the external mailserver ? Even an unsupported way ...

I have a postfix running on a BSD for 6 years never had an issue.
We do it the same way as plesk with a database. I don't want to use anymore the mailservices of plesk servers. But I don't want to force users as well to having to use another external control panel just for their mail.

two ways coming in my mind:
1. You could disable mail on the pleskserver for that domain and set the MX Record to another target. (best solution)
2. transport maps of postfix. But I don't know if Plesk rewrites it.

But there might be some more like .foward files for each mail or relayhost entry

Offcourse the MX will point to the other host.
The problem is with the users written in another database.

If you're using a db for the maps on your postfix server then it shouldn't be too hard.

• Give your postfix db user permission to connect from the plesk box.
• Write a shell script (or scripts) on plesk box for modifying the postfix db on external mail server.
• Add the event triggers in plesk for mail account created, mail account deleted, etc.

You will also need to have the shell scripts modify the postfix maps on the plesk box directly in order to have it appear that mail is enabled on the plesk box even though it's really using an external server. I've only done this with plesk running qmail but I don't see why it wouldn't work with plesk running postfix.

This is exactly what plesk expand is supposed to do. Unfortunately expand isn't a viable option any more. If you try this out then post back here and let us know how it went.

or sync one-way the whole postfix db from Plesk server where you configure to the one where mail is running.