2.1.3.1.3 From

To generate a From header, clients set the PidTagSentRepresenting property group (section 2.6.5).

When generating a MIME message, MIME writers generate a From header by using the values of the PidTagSentRepresenting property group. The order of preference in that property group is as specified in section 2.1.3.1.1.

When generating a TNEF message, MIME writers SHOULD<13> also copy the values of the PidTagSentRepresenting property group to the attSentFor attribute, as specified in [MS-OXTNEF] section 2.1.3.3.17, and the attMsgProps attribute, as specified in [MS-OXTNEF] section 2.1.3.3.21, in the TNEF body part.