When trying to save an email using the "Save to Box" Add-in, the only option is .eml. This was an old format for outlook express. Once an email is saved in this format, it cannot be opened in Outlook client. There needs to be an option for .msg. Using the File > Save As option allows you to change the extention, but that is extra steps. Outlook web will allow you to open the saved email, but you cannot save it as a .msg after the fact.