libytnef0-dev 1.9.2-2 (amd64 binary) in ubuntu bionic

 Yerase's TNEF Stream Reader allows you to decode application/ms-tnef
 e-mail attachments, which are usually entitled "winmail.dat" and are
 generally a file container format that is only readable by Microsoft
 Outlook. Some TNEF streams also include RTF-formatted data.
 .
 libytnef0 is the support library that exposes these functions to other
 programs. The ytnef program is the frontend for this library, so you should
 probably install that (included in ytnef-tools) if you want to take advantage
 of it.
 .
 These are the development headers for libytnef0.