Since you cannot send e-mail to customers who have NOT registered, you can still establish a new Unregistered group of Customers and populate that by hand (taking care to check their newsletter box), then use Sales / Mail to select those recipients as a group and Send. If there are too many to insert manually, then you may be obligated to use newsletter or bulk mailing options of your mailserver, so as not to commit spamming. You may also be obligated to provide unregistered recipients (although also registered ones) the customary newsletter or mail opt-out link(s) at the bottom, so as to stay within bounds of law. (Odds are that you don't have a mid-ocean island country of your own, and would like to keep the local and national constabularies from arriving unless you called them yourself.)
It will not be a good idea either to think about OC as a mail client or to use OC as a mail client.
Last edited by butte
on Thu Apr 04, 2013 11:26 am, edited 1 time in total.