Humm ... A mail message consists, very basically, of an "envelope", i.e. the header with the From, To Date, ID, etc. and separated by a CRLF the mesage body.
Nothing in the standards says what has to go in the body (other than max length of lines and other "formatting" things): It may well be yet another message with its own headers+body.
It's not recommended to do so mainly because some UAs may get completely lost trying to interpret such a message, but that really is a problem of the UA programmer not the standards per se. That leads to yet another solution: use the "cite" conventions to offset the original so that it can't be interpreted as a message itself.
I have to recognize that I've never received (or sent) a forwarded message so I don't really know what most UAs do in this case. A simple test would be forwarding a message to yourself (on another account) and compare with the original to see what's done.