Hi @Laura Nelson , can messages sent to other external recipients successfully?
According to the information 'Your message wasn't delivered because the recipient's email provider rejected it.' : the person you sent mail to has banned mail from your address or domain. It could be because you have sent too much mail and they marked your messages as spam.
Use the link below to check if your IP or domain is in black list of any spam
https://mxtoolbox.com/blacklists.aspx
Please also provide the complete NDR message here, and remove the personal information.
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