I don’t know for others but in my case tnef encoded emails originate from different companies, mostly multinational ones with local branches here in Greece and I suppose the users are using old versions of outlook, or their IT has a policy of enabling tnef (I can’t say though, why someone would choose to have such a policy).
I understand that the effort to support a legacy technology may be disproportionate and I wouldn’t even mention it if a workaround could be found. For the time being, as I mentioned before, lookout-fix-version seems to do the job although incompatible with 60beta versions. I’m afraid though, that this will not be the case for 60 release or for next releases and, in any case, having incompatible addons installed is not a good practice as they can be the source for unpredictable problems.
Unfortunately, I don’t have the required knowledge to undertake the maintenance of this addon.
Anyway, closing this, if someone has to suggest a workaround (other than notifying the users to disable tnef) It would be more than welcome.