Manager

 

SPF alone is no longer a reliable authentication method. In 2025, major mailbox providers require DKIM and DMARC alignment for consistent inboxing. SPF only authenticates the envelope sender (Return-Path) and does not authenticate the visible From domain. Because of forwarding, third-party senders, and alignment rules defined in DMARC, SPF by itself cannot provide a stable domain identity.

1.SPF Evaluates the Envelope Address (Return-Path), Not the Visible

SPF authenticates the Return-Path (also called MAIL FROM, envelope sender, or bounce address), which is hidden from the recipient in normal email flow.

• The From: header (the address the user sees) is never checked by SPF.
• The Return-Path is set by the last sending server and is often something like This email address is being protected from spambots. You need JavaScript enabled to view it., This email address is being protected from spambots. You need JavaScript enabled to view it., or This email address is being protected from spambots. You need JavaScript enabled to view it..

Result: Even if SPF passes perfectly, it proves nothing about the visible sender identity.

Example:

A legitimate newsletter sent from Substack on behalf of This email address is being protected from spambots. You need JavaScript enabled to view it.:

Even if SPF passes, it only confirms that Substack’s infrastructure is authorized by substack.com. It does not authenticate acme.com. If the platform allows arbitrary From values, an attacker could set the visible From to the same domain while still authenticating only the platform’s Return-Path domain. SPF does not validate this mismatch.

2. SPF Breaks Completely on Forwarding

When an email is forwarded (common with This email address is being protected from spambots. You need JavaScript enabled to view it. → personal Gmail, or old corporate aliases), the forwarding MTA re-sends the message using its own servers.

• The original Return-Path is usually replaced or wrapped.
• The new sending IP belongs to the forwarder (Gmail, university, etc.).

This is known as the “forwarding problem” and has been documented since RFC 7208 (2014).

Key Breakdown with "Visual" Annotations (as if highlighted in Gmail):

• Original Flow (SPF Pass, not shown in final headers): Bank's server (198.51.100.1) sends to user's old email (e.g., This email address is being protected from spambots. You need JavaScript enabled to view it.). Bank's SPF record: v=spf1 ip4:198.51.100.0/24 -all → PASS because IP matches.
  
• Forwarding Step: University server forwards to Gmail using its own IP, but keeps original Return-Path (This email address is being protected from spambots. You need JavaScript enabled to view it.). Gmail then re-sends from gmail-smtp-in.l.google.com (142.250.110.27).
  
  
• SPF Fail Highlight: The Received-SPF: fail line is what Gmail flags in red/orange in the viewer. It explains: Bank's domain doesn't authorize Gmail's IP → FAIL. This triggers spam filters for 10–30% of such alerts.
  
  
• Why It Looks Like a Bank Alert: The "Subject" and "From" mimic a real Chase or Wells Fargo notification. In real cases, users report these landing in spam with a warning banner: "This message seems dangerous" due to the SPF fail.
  
  
• DMARC Note: If the bank uses DKIM (most do), DMARC still passes via alignment on the From header, but SPF-only senders fail entirely.

Many banks and financial institutions still see 10–30 % of their alerts land in spam when users forward to Gmail/Yahoo, when they only rely on SPF.

3.Common Real-World Scenarios Where SPF-Only Fails

Scenario	What Happens to SPF	Visible Sender Authenticated?
Marketing platforms (Mailchimp, HubSpot, Constant Contact, Brevo)	Return-Path = platform domain → SPF pass on platform, not your domain	No
CRM / Helpdesk (Zendesk, Freshdesk, Intercom)	Return-Path = zendesk.com, freshdesk.com, etc.	No
Email aliasing (Google Workspace / Microsoft 365 aliases)	Return-Path often becomes This email address is being protected from spambots. You need JavaScript enabled to view it. or similar when routed externally	No
Mailing lists (Mailman, Groups.io, Google Groups)	List server rewrites envelope sender → SPF fails unless SRS (Sender Rewriting Scheme) is used (rare)	No
Transactional email via Amazon SES, Postmark, etc.	Return-Path = amazonses.com → SPF pass only on Amazon’s IPs	No

 

In every case above, SPF passes, but DMARC SPF alignment fails because the From domain (yourdomain.com) does not match the authenticated SPF domain (hubspotemail.net, rsgsv.net, etc.).

Note: In some of the examples above, you can configure SPF alignment directly from their portals (for example SendGrid, Amazon SES, Zendesk). By default, these services use their own domain for the Return Path.

4. DMARC Alignment When SPF Is the Only Passing Mechanism

DMARC requires alignment on at least one of SPF or DKIM:

• SPF alignment: From domain, must match the SPF-authenticated domain (or organizational domain with relaxed mode)
• DKIM alignment: From domain must match the DKIM-signing domain (d= tag)

If you have no DKIM signature, and SPF is authenticating a third-party domain, SPF alignment fails → DMARC fails → message is subject to p=quarantine or p=reject.

This is exactly what happened when Google & Yahoo rolled out their February 2024 bulk-sender requirements: thousands of SPF-only domains suddenly saw massive spam-folder placement.

5. Why DKIM Is Required for Stable Domain Identity

DKIM signs the From, Subject, Date, and body with a private key belonging to your domain. The signature travels with the message regardless of forwarding or third-party sending.

• Forwarding does not break DKIM (unless the header is altered)
• Third-party senders can sign on your behalf (you provide them the private key or use their on-behalf-of signing)
• DMARC DKIM alignment works even if the sending IP belongs to Mailchimp/SendGrid/Zendesk

6. How Mailbox Providers Treat SPF-Only Domains in 2025

Major MBPs now downgrade reputation signals for domains that:

• Consistently pass SPF but fail SPF alignment
• Have no DKIM signature at all
• Exceed DNS lookup limits (see below)

That single record triggers >16 DNS lookups once all the nested includes are expanded → permanent error (permerror) → most receivers treat it exactly like “no SPF record at all.”

Microsoft explicitly stated in 2024 that “weak authentication” (SPF-only with misalignment) is a strong spam-filter trigger.

7. DNS Lookup Limits and SPF Flattening Nightmares

SPF has a hard 10-lookup limit (RFC 7208). Many organizations that “include” multiple third-party services (SendGrid, Mailchimp, Marketo, Zendesk, status pages, etc.) quickly hit this limit and get “permerror”.

Common real-world SPF record that breaks:

v=spf1 include:_spf.google.com include:spf.mandrillapp.com include:mail.zendesk.com include:servers.mcsv.net include:spf.protection.outlook.com ~all

→ 12+ lookups → permerror → treated as SPF fail by most receivers.

8. Case Studies of SPF-Only Catastrophic Failures (2023–2025)

1. Major European retailer (2024)

Used SPF-only + SendGrid. After Google/Yahoo changes, 40% of promotional mail went to spam. Fixed only after implementing DKIM.

2. US healthcare provider

Zendesk helpdesk emails (SPF-only) are consistently marked as phishing by Microsoft 365 because of SPF misalignment. After adding DKIM signing via Zendesk’s feature, delivery jumped from ~60% inbox to 99%.

3. Well-known SaaS company 

Published SPF record with 14 includes → permerror → treated as no SPF → sudden drop in deliverability in October 2024. Took weeks to clean up.

Conclusion: SPF-Only Is No Longer Viable

SPF solves one problem: authorizing the server that sets the Return Path. It does not authenticate the domain shown to the user, and it does not survive forwarding or domain misalignment. Modern mail systems evaluate domain identity through DKIM and DMARC, not through SPF alone. Because of this, SPF-only domains produce unstable authentication results and inconsistent delivery.

To maintain a stable domain identity and pass DMARC:

1. Enable DKIM signing on every sending system.
2. Publish a DMARC policy and review reports for alignment failures.
3. Keep SPF simple and below lookup limits.
   
   

SPF should be treated as a transport-layer authorization method. DKIM provides the domain-level identity required by DMARC and by current mailbox provider enforcement. Without DKIM, a domain cannot maintain predictable authentication or reliable inbox delivery.