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I've now added this case as a doctest for decode_header in
rfc2822.py and this works. The problem is *not* in the
decoding of the header: The problem is in the generated
error message. Seems the error message gets appended
verbatim to the message text which is encoded in quoted
printable. The original header contained a "," instead of
";" for separating the two set-commands. But the resulting
error message suggested something else.
I've changed the title of this bug-report accordingly.