mail.setReferences syntax error
mail.setReferences('References: 1*<'+String.valueOf(b2bcid)+'> CRLF');
if I'm using this in my outbound messages class I'm getting error as
System.EmailException: SendEmail failed. First exception on row 0; first error: INVALID_MESSAGE_ID_REFERENCE, References header is not valid.: 
Class.WPOSP.B2BRespondtoinquirycontroller.sendResponse: line 59, column 5 External entry point
in the above code String.valueOf(b2bcid) is my record id. I was wondering what will be the syntax of mail.setReferenes method.
I also looked at http://tools.ietf.org/html/rfc2822#section-3.6.4 but couldn't understand the syntax.
Could you please help me in this.