Gateways and transports that connect to internet based systems, those that use SMTP, use the value of the MessageID SMTP header and the PR_TNEF_CORRELATION_KEY property to implement TNEF correlation.
The value of the MessageID header of the outbound message should be copied to the PR_TNEF_CORRELATION_KEY property and encoded in the attMAPIProps attribute of the TNEF stream. Note that PR_TNEF_CORRELATION_KEY is a binary property, while the MessageID is a string; the null terminator should be included in the copy and in the comparison.
This technique is used by all Microsoft software that connects MAPI-based messaging systems to the Internet, such as Microsoft Mail 3.x and Microsoft Exchange Server. This technique should be used by any SMTP gateways and transports that connect to systems that support MAPI clients in order to maximize interoperability.