You can either do a Partial restore from an eM Client backup.zip file or you can do a partial restore from the eM Client hidden database folder. See my link below for info.
Note: You will obviously have to first (extract either your eM Client backup.zip file with the missing email from your Macrium Reflect image), or (extract your eM Client hidden database folder with the missing email from your Macrium Reflect image) before doing the below.