Filladapt mode is good but the Perl module Text::Autoformat is better at reflowing text for email. I cobbled together the following elisp to make that easy.

(defun rel-autoformatter (begin end rightmargin)
  "Run Text::Autoformat on region"
  (shell-command-on-region begin end
   (format "perl -MText::Autoformat -e 'autoformat({all=>1,right=>%d})'" rightmargin) t t )
  (message "done."))

(defun rel-autoformat (begin end)
  Text::Autoformat on region, RFC recommended right margin of 72"
  (interactive "r")
  (rel-autoformatter begin end 72))

(defun rel-autoformat-quote (begin end)
  "Text::Autoformat on region, right margin of 60, suitable for quoting replies"
  (interactive "r")
  (rel-autoformatter begin end 60))

(global-set-key (kbd "C-c f f") 'rel-autoformat)
(global-set-key (kbd "C-c f q") 'rel-autoformat-quote)

It might be nice to handle parameters other than right margin- and do it in a cleaner way than explicitly specifying them. I doubt I’ll get around to it since Text::Autoformat usually does the right thing for me with just the margin hint.