The References: header is used by mail-reading programs to include a reference to the original message in replies. This header must have as its value a copy of the original Message-ID: header field:

References: <200210141542.g9EFg2bb006638@wash.dc.gov>

Notice that the message identifier is wrapped in angle brackets, which causes it to look like an address.

The References: header should never be declared in the configuration file.