In early versions of Evolution, we sent meeting requests like this:

  multipart/mixed
    text/plain (the user can type stuff here)
    text/calendar

This turned out to not work with Outlook 2000 in Corporate Workgroup
mode (or OWA), so we switched to this:

  multipart/alternative
    text/plain (Always just the string "Event data")
    text/calendar

Then we discovered that *that* doesn't work with Outlook 2000 in Internet
Mail Only mode (although the original way did). Currently we send this:

  text/calendar

Which, it has been pointed out, sucks, because you can't add any
explanatory text, or attachments.

There is also another problem, which is that if you have evolution
set up to automatically pgp-sign messages, you end up with:

  multipart/signed
    text/calendar
    application/pgp-signature

which, like the first version, will not work until O2k CW. (It's
possible that it doesn't work in O2k IMO either.) We may want to
add a "don't pgp sign calendar messages" option.

Outlook XP doesn't have separate CW/IMO modes, and I'm pretty sure
both of the first two layouts work there, though we haven't tested
this much.