so if I take a backup today and then another backup in 4 weeks time and then want to restore the first backup followed by the second backup, is there a way to do this?
As an aside, I actually backup twice a week but I would like to use particular backups as an archive which I can restore on another machine if necessary.
I have a main machine where EM Client removes the messages from the server.
The restore does not add to what is already there, it will completely replace the database. So if you use the first backup, eM Client will then be exactly as it was when you made the backup. If you then restore the second backup, eM Client will be exactly as it was when you made the second backup.
What you can do instead is export the messages, and use that as an external archive to selectively import files on another computer. If you choose Menu > File > Export > Export to .eml files, you can export whole tree structures or just single folders.