Another easy solution to this would be to place a copy of the email body on the clipboard and allow the user to paste in whatever email client they prefer.
"C-x h M-w" already does the same thing but better. It copies the subject, email, body, everything. That way you do not need to copy 3 different things when emailing from a different email client. I use the C-x h M-w approach when emailing bug reports via Gmail.