When you open an archive then the Finder shows an additional file next to the archive, called "Mail Archiv.vdb.journal".
What is this file and what does it do?
Each database operation like adding an email consists of smaller steps. If Mail Archiver would crash in the middle of one of these steps the archive would be in an inconsistent state. Therefore, the file protects you against data loss.
When you close the archive then the file should be deleted by the app because then everything should be written to the database.