2.2.169 [RFC6047] Section 2.5 Content-Transfer-Encoding Header Field
The specification states that a transfer encoding SHOULD be used for iCalendar objects containing any characters that are not part of the US-ASCII character set.
Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013
Outlook exports iMIP data with a Content-Transfer-Encoding value of "7bit" if the text is comprised solely of US-ASCII characters, and "8bit" otherwise.
On import, Outlook ignores the Content-Transfer-Encoding header and always imports iMIP data as UTF-8.