Without seeing exactly what you have done, it seems to me that you should have a separate layout for printing, and a script addition which looks for an empty email field. Upon finding that field empty, it will print the letter layout instead of sending the email.
It's the script addition that's causing the problem for me at the moment
though just before notification arrived that you'd responded, I think I had a breakthrough
Will post if it works
If not, will post with a further plea :-)
Thanks for your response