2.2.150 [RFC5546] Section 5.2.1 Cancellation of an Unknown Calendar Component
The specification suggests that messages that cannot be correlated that also have a non-zero sequence number be held and not discarded. Implementations MAY age them out if no other messages arrive with the same UID property value and a lower sequence number.
Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019
Outlook permits recipients to retain iTIP messages. The default behavior after an iTIP message is manually processed by the user is to move it to the "Deleted Items" folder, where the user can still interact with it.