2.1.3.2.13 Categories

To generate a Keywords header, clients set the value of the PidNameKeywords property ([MS-OXCMSG] section 2.2.1.17). The type of the PidNameKeywords property is multiple strings; each category SHOULD be mapped to a single keyword.

MIME writers SHOULD copy each subvalue of the PidNameKeywords property to a separate keyword in the Keywords header, with a comma (U+002C) and space (U+0020) separating each keyword. To avoid conflict among different sets of categories in different organizations, MIME writers can drop the PidNameKeywords property instead of copying it to the Keywords header.