I have now looked at my data dll and found the following there:
SIGNMESSAGE@F12NETEMAILSEND12STRINGTHEORYsbsbsb @9061
the prototype in netemail.inc is (which looks right?):
SignMessage Procedure(StringTheory pMessageBody,String pMimeBoundary1, String pMimeBoundary2,String pRootBoundary),Long,Virtual
I cannot find any other reference to SignMessage so I am not sure where the requirement for just sbsb comes from...