SMTP Server responded: failed | Domain contains control or whitespace

I'm sending an email to a client and have gotten the 'Subject' in red at the top of my New Transactional Email window.

What does it mean? Is it fixable?

Thanks!

And in sending the same email to another user, I go the same first part, with 'Local address contains....'

???

Can you please provide a screenshot or error message of what you're experiencing?

I'm seeing in this image that there's an error in your customer's email address.

You can delete the space between the '@' and the 'g' in your 'To: address' to fix this issue and please let me know if the error message still persists.