Since Outlook is one of the mailers that generates the non-RFC-compliant headers, it doesn't surprise me all that much that it can't interpret the RFC compliant headers correctly.

I'm not sure there is anything we can do here.

I suppose someone could do a survey of mail clients and document which ones can handle which style of parameter encoding.  If it turns out more handle the "wrong" way than handle the "right" way, we could consider adopting to the de-facto standard, although I won't like it much :)

(There is also a possibility there is a bug in our RFC compliance, but this is the first problem report I've seen.)