If you are using the feature Show in Mail in order to open an email from within Daylite in Apple Mail, you may receive the error message above. This error message indicates that the original email is no longer available in Apple Mail and therefore cannot be opened. As it is not possible to open Daylite's copy of an email in Apple Mail (which is beyond Daylite's control), Show in Mail works only if the original email is still stored in Apple Mail.
If the original email is no longer available in Apple Mail, alternatively you can right-click the email and select Email Selected Email from the contextual menu in order to open it in Apple Mail anyway.