Don’t know if anyone else has experienced this - sometimes when pressing Ctl-A to select all the text in an email, emClient will select all messages instead.
If you don’t notice this and then copy and paste, or worse cut or delete then you have lost all your messages! This just happened to me.
I just tried to recreate several times and was unable to replicate.
I am using current version 8.0.3318.0, Windows 10
I had a very helpful direct response from emClient who suggested I might be pressing Ctl-Shift-A which archives any selected messages - this could definitely explain it and worth bearing in mind for the future.
The keyboard shortcuts can be changed from the settings menu.