2.2.2 [RFC5545] Section 3.1.2, Multiple Values

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V0011:

The specification states that multi-valued properties are generally encoded by creating a content line for each value, including the property name. It also describes an alternative encoding, a single content line with the multiple values separated by a COMMA character.

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

On export, Outlook always uses the single content line encoding method. On import, Outlook can parse either method, or a combination of both.