Hi @Susan Dodds
Do you have an Exchange hybrid deployment and all outbound emails are routed through Exchange Online to internet?
I have owa.contoso.com as my email domain and is my custom domain in the microsoft portal. I do not have just contoso.com as a domain on my admin portal.
According to the NDR message from Google, your email domain is contoso.com.
Have you enabled DKIM for contoso.com following this link?
Do you have your on-premises Exchange server's ip address in the SPF record?
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