2.2.13 [RFC5545] Section 3.2.8, Format Type

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V0020:

The specification describes the optional FMTTYPE parameter.

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

Outlook only exports an FMTTYPE parameter for the X-ALT-DESC property ([MS-OXCICAL] section 2.1.3.1.1.20.27), and the parameter's value is "text/html".

On import, Outlook ignores the FMTTYPE parameter on any properties other than X-ALT-DESC and ATTACH. Furthermore, Outlook ignores any X-ALT-DESC properties with an FMTTYPE parameter other than "text/html" (case-insensitive).