it will bounce "message has lines too long for transport".

Therefore
C-c C-f runs the command gnus-summary-mail-forward
should have some checks that will warn of long lines.

Like: "Long lines found. Fix?"

And also one of these
if ARG is 1, decode the message and forward directly inline;
if ARG is 2, forward message as an rfc822 MIME section;
if ARG is 3, decode message and forward as an rfc822 MIME section;
if ARG is 4, forward message directly inline;
should enable us to fix the problem.