So it's only your computer that's doing this? No other users, no other devices, etc.. are doing this? Have you tried a different browser perhaps? I'm not sure what to tell you as it's server side so what computer or device you use would be completely irrelevant. Try registering a user then navigating to CB > User Management and clicking their checkbox. Next click Mass Mailer and see if they can receive that email. Also try using a different email address if possible.