2.2.3.2.28 PidTagTransportMessageHeaders
MIME readers SHOULD copy headers from an inbound message to the value of the PidTagTransportMessageHeaders property ([MS-OXPROPS] section 2.1051). The rules for including and excluding headers are as follows:
Headers whose names do not begin with the reserved name prefixes "X-MS-Exchange-Organization-" and "X-MS-Exchange-Forest-" SHOULD be copied to the PidTagTransportMessageHeaders property.
Headers specifically mentioned in section 2.1.3.2 SHOULD be copied to the PidTagTransportMessageHeaders property, even if their names begin with the reserved name prefixes "X-MS-Exchange-Organization-" or "X-MS-Exchange-Forest-".
Headers that are not specifically mentioned in section 2.1.3.2, and whose names begin with the reserved name prefixes "X-MS-Exchange-Organization-" and "X-MS-Exchange-Forest-", SHOULD NOT be copied to the PidTagTransportMessageHeaders property.
This property value SHOULD be set only by MIME readers upon delivery of a message from SMTP, in which case it SHOULD be set to the header of the top-level message (with exceptions as already specified). A MIME reader MAY<140> also set the PidTagTransportMessageHeaders property ([MS-OXOMSG] section 2.2.1.61) when downloading messages via IMAP4.