Convert rcube_message back to raw message string

[ktwalrus]

I'm a newbie trying to use the Roundcube Framework to fetch all messages from specific mailbox, reformat/change the from, to, cc, subject, and some other headers, and then convert rcube_message back to a raw message string for sending on to another mailbox.

I can't find a function to convert the internal rcube_message back to a string (or save it to a file).

How do I do this?