+PH/14 Messages created by the autoreply transport now contain a References:
+ header. This is constructed in accordance with rules that are described
+ in section 3.64 of RFC 2822, which states that replies should contain
+ such a header line, and section 3.14 of RFC 3834, which states that
+ automatic responses are not different in this respect. However, because
+ some mail processing software does not cope well with very long header
+ lines, no more than 12 message IDs are copied from the References: header
+ line in the incoming message. If there are more than 12, the first one
+ and then the final 11 are copied, before adding the message ID of the
+ incoming message.
+