Export / Import server mail database...

Hi, can any one help me.

I have a broken Mac OS X Yosemite Server and I need to make on him a clean install, while I can't use TimeMachine backup. The TimeMachine backup destroyed worser the server. I have all settings backed-up and I need now only backup my complete mail database with all emails. Does any one how to export the database from server and then import it correctly back to another server. I tried to make a copy and put it on a clone of existing server but it doesn't work correctly. Maybe i missed something to do.

Thanks.