LookOut Fix-Version is a plugin which allows Thunderbird to interface with Microsoft’s mail tools by decoding metadata and attachments encapsulated/embedded in a TNEF (Transport Neutral Encapsulation Format) encoded attachment (aka winmail.dat).
Transport Neutral Encapsulation Format or TNEF is a proprietary email attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most often named winmail.dat or win.dat, and has a MIME type of Application/MS-TNEF. The official (IANA) media type, however, is application/vnd.ms-tnef. Source: https://en.wikipedia.org/wiki/Transport_Neutral_Encapsulation_Format
Unfortunately I had to stop working on Lookout a while ago because my employer switched us back to Outlook leaving me with no company time to maintain the project any more. The last major release of Thunderbird I received a lot of help from the TB team's John Bieling to bootstrap a workaround to get things rolling again, now Thunderbird 91 is coming and it's another case of the old xul add-ons not working in the webextension world.
After receiving no takers to my maintainer request over on the mailing list I have decided if I am unable to locate new maintainers I will archive the project on Github. I hope the Thunderbird devs finally assign Bug 77811 to someone as I know that proper integration for decoding TNEF emails was added to the roadmap, however in the meantime you are interested in updating the project please let me know as I'll actively be monitoring the Github project and the issue calling for new maintainers