Magic!

How did it get fixed? Did you fix it or is it something I can tweak on my end?

No, I did nothing. And now just the same happens. Crazy.
Please have a look. It is about an early-bird offer that ends tomorrow.

Our team is investigating this and will get that error resolved as quickly as possible. Thanks for your patience!

Thanks Adam! 

Hi again Adam,
I've just noticed that we received a bunch of orders from the email I attempted to send this morning which is weird as on my Promote dashboard/ under Emails, it lists that email as unsent still.
More interesting is that in the Insights/Marketing/Emails is lists this mornings email but with YESTERDAY'S date. That must be creating a huge issue...

I'm not sure of the specifics behind what needed to be fixed, although it definitely sounds like it must have actually sent the email even though it displays the reverse. That should get corrected once the issue is fixed and any changes are processed.