Adobe Sign keeps saying model release emails have bounced. How do I fix it?

Hello everyone! 

So I was uploading some images to adobe stock and I was preparing new release forms for 2 different models. I went through the usual process but it keeps saying the status for each of them is "Bounced" - i know the email addresses are correct, I've tried restarting my device, logging in and out, using different browsers, trying to create a new release directly and pressing the create new release button when preparing the images for submission. It's happened before and I gave up on that specific release because the image wasn't that good to begin with and then it worked for other people/emails later so is it a case of Adobe Sign acting up or is there something I can do to fitx this myself? The email addresses are hotmail.co.uk if that's relevant.

Thank you - have a nice day!