I see your point Stuart and I'm gonna try it as soon as it is possible for me, but right now I needed an ASAP sollution and the trial version of the LDAP Administrator did the trick. It was as simple as I wrote it. I just made a filter to export all the users who had the mail=*@#####.## in LDIF. I checked the file with Notepad++ and looked pretty nice. Imported the file to the server with the same app and suprisingly it was a 99,99% success. If the export is only to have a backup and you're gonna import the users to a server with the same setup, I think it works fine.
But I'm deffinetly going to try the bulk user thingy. THanks a lot! Shall I mark the topic solved?