Somehow you will receive an email contains with only winmail.dat attachment when it is sent from Microsoft Outlook client.

To handle this you will need TNEF command to extract the content.

$ tnef --number-backups --save-body=filename_to_be --body-pref=H --directory=/path/to/output/folder /path/to/winmail/file 2>&1

--number-backups; there could be some attachments with the same name, so this will put a digit at the end of each of duplicated files, otherwise it will be overwritten e.g. file, file.1, file.2

--save-body; this will save the email body to a file with the specified filename; filename_to_be

--body-pref; the email body could be in (R)TF, (T)EXT, or (H)TML format. But let’s get only HTML format if any.