Direct from rfc1847:
"
A MIME agent that includes support for this framework will be able to
recognize a security multipart body part and to identify its
protected data and control information body parts. If the value of
the protocol parameter is unrecognized the MIME agent will not be
able to process the security multipart. However, a MIME agent may
continue to process any other body parts that may be present.
"
The last sentence is the meaningful one. Please pass the bug onto the slrn folks.
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