2.2.16 [RFC3501] Section 5.2, Mailbox Size and Message Status Updates
The specification states: "At any time, a server can send data that the client did not request. Sometimes, such behavior is REQUIRED. For example, agents other than the server MAY add messages to the mailbox."
Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019
Outlook handles unrequested data from the server and from agents other than the server.