acmecorp.com MX pointing to Google Workspace
m.acmecorp.com MX pointing to Office 365
When you have a split domain (mailboxes in both Office 365 and Google Workspace) and you are using a subdomain to redirect to Office 365, some messages will bounce or not get delivered. Google looks up the MX record for the subdomain, and then resolves that using both IPv4 and IPv6. When delivering mail via IPv6, SPF or DKIM must pass for Office 365 to accept it. If they do not, the message is rejected, and you will errors in the Google Workspace Email Log Search.
Solution is for Office 365 to not resolve the MX host to an IPv6 address.
Connect-ExchangeOnline Disable-IPv6ForAcceptedDomain -Domain m.acmecorp.com
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