2.2.21 [RFC3501] Section 6.3.11, APPEND Command
The specification states: "If the mailbox is currently selected, the normal new message actions SHOULD occur. Specifically, the server SHOULD notify the client immediately via an untagged EXISTS response."
Microsoft Exchange Server 2007, Microsoft Exchange Server 2010, Microsoft Exchange Server 2013
If the mailbox is currently selected, then Microsoft Exchange Server does not immediately send an untagged EXISTS response. Instead, Microsoft Exchange sends an untagged EXISTS response upon successful completion of APPEND within the currently selected mailbox.