2.1.3.2.14 In-Reply-To Message ID

To generate an In-Reply-To header, clients set the value of the PidTagInReplyToId property ([MS-OXOMSG] section 2.2.1.13).

MIME writers SHOULD<39> copy the value of the PidTagInReplyToId property ([MS-OXOMSG] section 2.2.1.13) to the In-Reply-To header.