2.2.24 [RFC3501] Section 6.2.3, LOGIN Command


The specification states: "A server MAY include a CAPABILITY response code in the tagged OK response to a successful LOGIN command in order to send capabilities automatically. It is unnecessary for a client to send a separate CAPABILITY command if it recognizes these automatic capabilities."

Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019

Outlook sends a CAPABILITY command regardless of whether the server sends the capabilities automatically.