Invalid key file format

I’m using MailStore Home on a laptop running Windows 10 Home (fully up-to-date )and had a power failure during a backup.

When I try to start the program I’m now getting the folowing error message :-

MailStore Home was unable to open the archive. Details: Invalid key file format

Looking at the backup folder, everything looks OK apart from the fact that the MailStoreFileGroup.key file is ZERO bytes long.

The actual data backup .dat files look OK as does the MailStoreFileGroup.fdb file.

Looking at other posts, the .key files are (not surpisingly) important and the only other one showing - MailStoreMaster.key is 3 kb in size. For safety reasons I haven’t touched this file.

Is it possible to recreate the MailStoreFileGroup.key file or take any other action which would allow the program to run ?

Any advice wpould be gratefully received.

Have you tried re Archiving your email …I have just tried this and a new FDB file has been created

Forgot to add …all in the previous attached jpg have today’s date

Just in case you did not think of it: did you try to Restore Previous Versions on the MailStoreFileGroup.key file (the one which is now 0-byte long)?

Thanks for the suggestion - how would I do that ? Please bear in mind that I’m running MailSrore Home on a stand-alone laptop and not on a server.


You said you encountered the MailStoreFileGroup.key file but it was empty (0 bytes long). In Windows Explorer, going to this 0-byte file, right click gives you a menu. On the middle of it (just below Open with…) is the Restore Previous Versions option. Alternatively, it is a snap on the Properties dialog for the file.

This Windows feature combines every backup source your computer is aware of. You are right that if the file is stored at a file server (and using Win10 Pro), you might see more options. But on Win10 Home, it might access the File History archive, and the Windows Backup’s which could have been registered (as well as Restore Points, but I feel this will not be useful in your case).

Thanks again for the suggestion, but unfortunately I don’t have File History running as part of W10 Home.

Hi @PRR_SoT ,

the MailStoreFileGroup.key file is very important.
It is a definition file to the encryption, since the archive files are encrypted to prevent any content manipulation.
Without a working key file, MailStore Home cannot decrypt and open the archive. It is unusable now.

In MailStore Home, you find a management option “Backup to USB/HDD” to create backups. It would have been wise to use this once in a while.
Then you’d have had the chance to recover an older key file from a backup, since it rarely ever changes.

It cannot be recreated now, that would mean that the encryption could be reverse-engineered, something that would make encryption obsolete.
When there is no useful key file you can recover, the archive is unreadable and lost.