2.2.178 [RFC6047] Section 5.1 Use of Content and Message IDs
The specification states that it is strongly recommended that iMIP implementations include all referenced messages and body parts in a single MIME entity.
Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019
Outlook does not export MID URIs. Outlook only exports CID URIs for attachments that are included in the same MIME message as the iCalendar object.
On import, Outlook ignores all MID and CID URIs. Attachments in the same MIME message as an iMIP message are attached to the resulting meeting request, response, cancellation, or counter-proposal.