I'm working on a mailing system for a scheduling server and have managed to get it working with one quirk which I don't quite understand. The system comes out with an email containing calendaring data as content which only MS Outlook ignores (all other services and clients that we've tried accept the header) but I can't quite see why and would be grateful for any pointers as to what to investigate to solve this. From the headers I've seen, the text/calendar heading is accepted but the data is being ignored.
The headers are built here:
and then sent via a thread.