2.2.176 [RFC6047] Section 4.5 Multiple Mixed Components
The specification provides a sample message to show how different component types must be encapsulated in separate iCalendar objects.
Microsoft Office Outlook 2007
It is assumed this section is about iMIP messages with the following structure:
Office Outlook 2007 does not export multiple iMIP parts in the same MIME message.
On import, Office Outlook 2007 only searches for iMIP parts in the first child (and its descendants) of a multipart/mixed MIME part. The second text/calendar part of this message would be treated as an attachment. Furthermore, since Content-Disposition is set to "attachment" on the first text/calendar part, it is also treated as an attachment.
Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019
Outlook 2010, Outlook 2013, Outlook 2016, and Outlook 2019 behave identically to Office Outlook 2007, with the following exception:
Outlook 2010, Outlook 2013, Outlook 2016, and Outlook 2019 search in all children of a "multipart/mixed" MIME part for iMIP data. If multiple children contain iMIP data, all but the first are treated as attachments. However, since Content-Disposition is set to "attachment" on the first text/calendar part, it is also treated as an attachment.