Usually, a saved .msg file from some e-mail client is HTML based, so you can view, edit or re-save it using any Text/HTML editor. For some specialized MSG files you may need proper conversion utilities or use the original program from which that particular .msg file type originates.
Convert Saved email message to Simple text file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert msg to txt on Windows
A flexible email viewer
Convert from msg file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.