New brand, does not send test mail. Gets stuck sending mailing at 99%. Other brands work fine!

I recently created a new brand for a different product we are marketing.
I created a campaign, with a simple email.
When I try test send, it stated it has been sent. I however dont receive it.
When I try send a campaign, it gets stuck at 99% and says its sending...
I checked Amazon, no issue. In fact I sent a big campaign with the same set up yesterday in another brand and that went out, no problems.
I hope you can help!