The urn:schemas:httpmail: namespace defines many fields similar to those found in the urn:schemas:mailheader: namespace and comprise core properties for messages. However, within this namespace, string fields are returned with all characters that were encoded per RFC 1522 decoded into UNICODE characters.
This namespace also contains additional fields not found in urn:schemas:mailheader: that are routinely used to render messaging information to clients through a web browser or user agent.
Use these fields to render messaging information within web browsers or user agents (UAs), especially when characters within the various message headers contains are not US-ASCII, or when you are unsure what character sets to expect. All fields within the urn:schemas:mailheader: namespace are always returned as US-ASCII characters, with any non US-ASCII characters encoded using the mechanism described in RFC 1522.