We are having major issues with some new Private Domains when it comes to delivilibty - the biggest issue being blocked bounces that are having 'Authentication' issues. The SAP that was originally made for that MID is not experience these same issues.

The private subdomains are purchased through Salesforce so all of the needed authentication should be in place. Looking in google postmasters tool the DKIM and DMARC both get 100% success rate and the Domain Reputation is at a medium.

Salesforce is telling me that my send volume for my MID is too small for a private IP Address but that address is also used on another MID that has a very large send volume so I don't see how that could be the issue.

I think the issues lies with the SPF which success rate is always 0%. How do I fix this issue? Or is there any ideas as to what the issue could be?