SPF failures in SFMC, even when seemingly passing, are multifaceted. Root causes include unaligned SPF passes (passing for SFMC but not the client domain), discrepancies due to different MIDs/IPs, incorrect SPF record syntax, not including Salesforce's IPs or the necessary `include` statement for SFMC, SPF alignment issues, misconfigured bounce domains, SenderID interference, exceeding DNS lookup limits, and the fundamental limitation of SPF in fully protecting the 'From' address. The fix involves verifying SPF syntax and domain alignment, correctly configuring bounce domains, including necessary senders (especially Salesforce), flattening SPF records, deprecating SenderID, thoroughly testing configurations, and understanding the need for DMARC in conjunction with SPF.