Does this email address using a new domain name? If so, you need to be the owner of this domain and have verified this domain in your Office 365.
After that, if you still cannot sync this email address to AAD, you can confirm with Azure AD connect team.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.