Processing multiple MSG files is easy with MSG Viewer for Outlook. First select all of the MSG files you want to convert, and drag them right onto the MSG Viewer for Outlook app (or right-click and choose Open With → MSG Viewer for Outlook). This will automatically start processing your MSG files into the proper format.
Also, when MSG Viewer for Outlook is batch-processing, your email client will not be automatically launched – this is to prevent overloading your email software. Once the conversion is complete, files are saved in the same folder where they were originally located.
Pro tip: All files must have the .msg, .dat, .eml, or .oft file type to be processed in bulk. If you have included other file types in your selection, you will not see the option to Open With → MSG Viewer for Outlook (or similarly, a red "X" will appear if you drag/drop within the app).