e-Newsletter error - blank email from "Root User"

I've just setup the plugin (v2.2.3) and configured it to use SMTP.
I created a basic test newsletter using the iLetter template and used the preview function... ALL GOOD (subject, sender, content all perfect).

HOWEVER, upon actually sending the newsletter to 2 subscribed users they received an email with no subject, blank body (apart from a "View this email in browser" link), and the sender of the email is "Root User" (no email address, just the name)

Any ideas?