2.2.2 [RFC3501] Section 2.2.2, Server Protocol Sender and Client Protocol Receiver
The specification states: "Server data MAY be sent as a result of a client command, or MAY be sent unilaterally by the server. There is no syntactic difference between server data that resulted from a specific command and server data that were sent unilaterally."
Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019
Outlook can handle unrequested responses.
The specification states: "Server data SHOULD be recorded, so that the client can reference its recorded copy rather than sending a command to the server to request the data. In the case of certain server data, the data MUST be recorded."
This part of the specification does not state which server data MUST be recorded. A thorough reading of the specification reveals that the client MUST record the following server data:
Mailbox size updates (section 5.2 of the specification)
Updates from the FLAGS response (section 7.2.6 of the specification)
Updates from the EXISTS response (section 7.3.1 of the specification)
Updates from the RECENT response (section 7.3.2 of the specification)
Updates from the EXPUNGE response (section 7.4.1 of the specification)
Office Outlook 2007, Outlook 2010, Outlook 2013, Outlook 2016, Outlook 2019
Required recording of certain server data: Outlook records only three of the five server data that are required to be recorded. The following are not recorded:
Updates from the FLAGS response ― Outlook ignores the FLAGS response, as specified in section 2.2.46 of this document.
Updates from the RECENT response ― Outlook ignores the RECENT response, as specified in section 2.2.47 of this document.
Recommended recording of other server data: Outlook maintains a local cache of all user data and some meta-data (UIDs, hierarchy characters, etc.), thereby reducing the likelihood of the same data being repeatedly fetched from the server.