2.2.3.1.3 From

To set the values of the properties in the PidTagSentRepresenting property group (section 2.6.5), clients MUST set the From header value, as specified in [RFC2822].

MIME readers MUST set the value of the PidTagSentRepresenting property group to the value of the first e-mail address component of the From header (which can contain multiple e-mail addresses). If the From header contains multiple addresses, the first address MUST be used; the others are ignored.

When reading TNEF messages, MIME readers SHOULD<109> use a From header value specified in MIME format in preference to the attSentFor attribute, as specified in [MS-OXTNEF] section 2.1.3.3.17, or the PidTagSentRepresenting property group values of the attMsgProps attribute, as specified in [MS-OXTNEF] section 2.1.3.3.21, specified in a TNEF message, except for messages attached to a TNEF message, where a MIME header does not exist.