Outlook calendar invites being converted to plain text


Knowing full well that this is outside the scope of the forum, I beg your indulgence.

Based on suggestions received here, I configured my outbound email to be delivered via MandrillApp. On the whole, it works very well. However, I believe that this approach has also caused a problem: when I send a calendar invitation from Outlook, it is received as plaintext.

I won’t bore you with the details, but I have been able to do a fair amount of testing, using one instance of Outlook and several different email servers, and all of the evidence seems to suggest that Mandrill is the culprit. Has anyone else seen this behavior? More importantly, do you know a way to resolve the problem?

Before you ask, all other attachments are delivered properly - it is just iCalendar attachments that seem to be problematic.