The non-commercial version of Ricemail has a known problem whereby, if
you forward a message whose header has lines longer than 80 bytes
(typically, a "Received:" line), the forwarded header contains an
incorrectly folded version of the long field which results in the syntax
error you are observing. I don't know of any straightforward solution,
other than TELL MAILER SET PUNCH, which tells LMail to fold all lines in
your message at 80 bytes. But of course this means you can no longer
receive messages with long lines.
Eric