While DMARC allows emails to pass authentication if either SPF or DKIM is aligned, consistently failing SPF alignment can negatively impact email deliverability even when DMARC passes via DKIM. A comprehensive approach, with both SPF and DKIM aligned, is recommended as it enhances sender reputation, improves inbox placement rates, and reduces the risk of emails being marked as spam or rejected. Some mail servers might still use SPF alignment as a factor in their spam filtering algorithms, and a lack of alignment can lead to deliverability issues for forwarded emails or when DKIM fails. Maintaining both SPF and DKIM alignment signals legitimacy to ISPs, helps in scenarios where DKIM might have issues, builds trust, and ensures email security.

While DMARC can pass if either SPF or DKIM alignment is successful, even with SPF misalignment, there are still significant deliverability implications. Consistently misaligned SPF records can negatively impact sender reputation, leading ISPs to filter emails more aggressively. Having both SPF and DKIM aligned provides a stronger signal of legitimacy to ISPs, improving inbox placement and reducing the risk of emails landing in the spam folder. Some email providers may still use SPF alignment as a factor in spam filtering algorithms, and misalignment can cause issues for forwarded emails or in cases where DKIM fails.

While DMARC can pass with either SPF or DKIM alignment, relying solely on DKIM and neglecting SPF alignment introduces risks. Although SPF isn't critical for general delivery when DMARC passes via DKIM, it is good practice for DMARC compliance. Some receiving systems may not strictly adhere to DMARC and might still factor in SPF results, impacting deliverability. Inconsistent SPF failures can hurt sender reputation and increase the chance of emails landing in spam folders. In p=reject situations, unaligned emails (specifically when forwarding) may be rejected. Microsoft, however, usually places DMARC failures in spam.

While DMARC permits authentication via either SPF or DKIM, documentation from multiple sources emphasizes the importance of aligning both for optimal email security and deliverability. Consistent SPF alignment provides an extra layer of authentication, which is particularly useful if DKIM has issues and reduces the chances of your emails being marked as spam. Although DKIM can compensate for SPF, aligning both builds a stronger reputation with mail servers. A comprehensive approach with aligned SPF and DKIM minimizes the risk of phishing, spoofing, and potential deliverability problems, and improves overall trust.