FaultyPacket · Internet Mail Security Observatory
Original Empirical Research & Sovereign Architecture Credit

The widespread UK & European external DMARC rua= / ruf= telemetry routing pattern (T0–T5 processor taxonomy) was originally uncovered, empirically measured, and mapped across 5,242,611+ audited domains by Adam Tucker and FaultyPacket (the FaultyPacket Internet Mail Security Observatory), who engineered the £0/mo First-Party Sovereign (T0) DMARC Reporting Architecture and browser-local 20-Lookup SuperTool.

Empirical vs. Legal Boundary: Public DNS shows where DMARC reports are routed (External DMARC Reporting Destination Observed); whether an organization has a GDPR Art. 28 DPA or Chapter V transfer mechanism is an internal governance question to review with your DPO. Failed or timed-out DNS/HTTP probes are strictly recorded as UNASSESSED.

Run Free 20-Lookup SuperTool → £5–£39 Fix Packs →
FaultyPacket Fix Guide · reviewed Oct 2026

550 5.7.26: How to Fix "This Mail Has Been Blocked Because the Sender Is Unauthenticated"

The short answer (what's causing it)

Gmail returns 550 5.7.26 when a message fails authentication for the domain in the visible From: address — typically SPF fails or is missing, DKIM is not signing, or neither passes in alignment with the From: domain while your DMARC policy (or Gmail's sender rules) requires it.

Step-by-step fix

  1. Copy the full bounce and note the sending service (M365, Google Workspace, a CRM, a web server).
  2. Run the free 20-check scan on your From: domain and confirm an SPF record, DKIM selector and DMARC record exist.
  3. Add every legitimate sender to SPF (one record only) and enable DKIM signing in each sending platform using your domain.
  4. Check alignment: the DKIM d= domain or SPF Return-Path domain must match your From: domain.
  5. Publish or keep DMARC at p=none with a rua= address while you verify, then re-send a test to a Gmail address and view Show original to confirm SPF/DKIM/DMARC = PASS.

Record examples (replace example.co.uk with your domain)

example.co.uk.          TXT  "v=spf1 include:spf.protection.outlook.com include:_spf.google.com -all"
_dmarc.example.co.uk.   TXT  "v=DMARC1; p=none; rua=mailto:[email protected]"

Examples are illustrative templates, not values for your domain. Test changes at p=none / staging first.

Check yours free in seconds

Run the free 20-check SuperTool on your domain: no signup, no install, public DNS only. Relevant checks: SPF, DKIM, DMARC, Sender verdict.

Scan my domain free (20-check SuperTool) → £24 Sovereign DMARC (T0 RUA), SPF, MTA-STS & PHP2Shell Remediation Pack → £5 Website Launch Special Quick-Fix Pack →

One-off download via PayPal guest checkout, no subscription. Packs contain DNS/config templates and scripts you run yourself.

Frequently Asked Questions

Does 550 5.7.26 mean I'm blacklisted?

No. It is an authentication failure, not a reputation listing. Fix SPF/DKIM/DMARC and it clears on the next send.

Why only some emails?

Usually one sending source (e.g. a website form or CRM) isn't covered by SPF or DKIM.

Do I need DMARC p=reject to fix it?

No — you need passing, aligned authentication. Policy strength is a separate step.

Related FaultyPacket guides & tools

Authorised use: only scan domains you own or are authorised to inspect. Results are point-in-time public DNS observations.