Read the evidence.
Fix the cause.
Mailybox turns DNS records, message headers, SMTP rejections and provider policy into one evidence model — so you can see why delivery failed, test the change, and verify the result.
Four evidence layers evaluated together, not as isolated pass/fail checks.
Start with the problem, not the protocol.
Each tool begins with a real question: is this domain protected, why did delivery fail, is this IP listed, what does the record need to say.
DMARC Record Checker
Find the DMARC policy that actually applies, validate its core tags and expose multiple-record or RFC 9989 fallback conditions.
open →Mail Failure Doctor
Turn a bounce, NDR or SMTP rejection into a precise diagnosis and a prioritized fix plan.
open →SPF Dependency Graph
Resolve SPF recursively, count DNS-triggering mechanisms and reveal the exact path to PermError risk.
open →Email Blacklist Check
Check whether a sending IP address is listed on 17 public DNS blocklists, read the listing reason, and confirm reverse DNS at the same time.
open →DMARC Record Generator
Build a syntactically valid DMARC record from plain-language choices, see what each tag does, and compare it with the record your domain publishes today.
open →Email Header Forensics
Decode authentication results, Received hops, identity changes and delivery delays from a raw header or .eml file.
open →Email Infrastructure Digital Twin
Map the public email control plane of a domain and expose dependencies, weak points and provider fingerprints.
open →Bulk DMARC & SPF Checker
Audit MX, SPF, effective DMARC policy, MTA-STS and DKIM selectors across up to 25 domains in one pass and get a graded posture table.
open →DKIM Inspector
Resolve a DKIM selector, follow delegation and inspect the published key instead of guessing from the root domain.
open →Start from the bounce, NDR or SMTP response and work backward to the cause.
SPF includes and DMARC discovery are resolved the way receivers resolve them.
Build a record, check its lookup budget, then confirm it propagated.
Observation is separated from inference; nothing is called a guarantee.
Email rules change. The diagnostics move with them.
Current standards and provider changes are built into task-specific analysis rather than left as static reference text.
Browse the field guidesInventory web Send-as, Gmailify and POP dependencies against Google’s published transition.
Trace organizational-domain discovery the way the current standard defines it.
Read current reporting metadata, sender volume and outcomes in one view.
Compare identity evidence when an email asks for new bank details.
Model the blast radius before an agent can read, send or forward.
Technical depth without the maze.
Guides explain the failure modes behind the tools and link every concept back to something you can verify.
Gmail 2027 Third-Party Account Changes: Audit Send As, POP and Gmailify Dependencies
Identify Gmail web workflows affected by the January 2027 third-party account changes and plan replacements early.
read →DMARC RFC 9989 DNS Tree Walk: How Organizational Domain Discovery Changed
Understand the current DMARC policy-discovery and alignment algorithm, including the eight-query bound and psd boundaries.
read →SPF Too Many DNS Lookups: Diagnose the Actual Dependency Chain
A practical guide to recursive SPF lookup pressure, PermError risk and safer remediation.
read →Business Email Compromise Thread Forensics: Find the Identity Break
A disciplined way to compare messages inside a business conversation without treating one suspicious field as proof of fraud.
read →Every analysis, one click away.
49 tools, 35 guides, 8 provider hubs and 27 error-code references.
Diagnose
- Mail Failure Doctor
- Email Infrastructure Digital Twin
- MX Record Lookup
- SPF Dependency Graph
- DMARC Record Checker
- DMARC DNS Tree Walk Explorer
- SMTP Conversation Analyzer
- DKIM Inspector
- BIMI Readiness Analyzer
- Email Provider Detector
- Email Dependency Discovery
- Reverse DNS & HELO Auditor
- One-Click Unsubscribe Validator
- Email Infrastructure Conflict Analyzer
- Bulk DMARC & SPF Checker
- Email Blacklist Check
- Email DNS Propagation Checker
Build & generate
Simulate
Forensics
Security
Developer QA
Guides
- SPF Too Many DNS Lookups: Diagnose the Actual Dependency Chain
- How to Read Email Headers Without Guessing
- Gmail Clipping: Measure the Payload Before It Hides Your Footer
- DMARC Alignment: The Identity Relationship That Decides the Result
- Email Bounce Root-Cause Analysis: Read the Stage, Not Just the Code
- A Practical Email Infrastructure Audit for Multi-SaaS Domains
- Gmail 550 5.7.26 “Unauthenticated Email” Fix
- Outlook 550 5.7.515: High-Volume Sender Authentication Diagnosis
- Bulk Sender Requirements in 2026: Build One Authentication Baseline Across Providers
- MTA-STS and TLS-RPT: Audit the Transport Layer, Not Just SPF and DMARC
- DKIM Selectors and Key Rotation: Verify the Key a Message Actually Used
- Email Forwarding, SPF, SRS and ARC: Why Authentication Changes in Transit
- One-Click Unsubscribe: Test the Final Message and the Suppression Workflow
- HTML Email Accessibility: Preflight the Things a Visual Preview Misses
- Transactional Email Chaos Testing: Prove Retries Before a Provider Incident
- Email Migration Risk: Map the Dependencies That MX Records Do Not Show
- Reverse DNS and PTR for Email: Verify the Sending Identity at the IP Layer
- Lookalike Domain Defense: Prioritize the Variants That Can Actually Send Mail
- Email Provider Detection: Why MX Alone Tells Only Half the Story
- BIMI Readiness: Check DMARC Enforcement Before Debugging the Logo
- DMARC RFC 9989 DNS Tree Walk: How Organizational Domain Discovery Changed
- How to Read DMARC Aggregate Reports Under RFC 9990
- TLS-RPT Failure Types: Diagnose Why Secure Mail Transport Broke
- Gmail 2027 Third-Party Account Changes: Audit Send As, POP and Gmailify Dependencies
- ARC Forwarding Analysis: Preserve Authentication Context Across Intermediaries
- Email DNS Change Review: Simulate the Blast Radius Before Publishing
- Email Dependency Discovery Before Migration: Find the Senders Nobody Documented
- EML Email Forensics Workflow: Reconstruct Identity, Transit and Message Structure
- Business Email Compromise Thread Forensics: Find the Identity Break
- How to Reconstruct an Email Incident Timeline from Raw Messages
- Email Infrastructure Ownership Conflicts: Find Authorization Debt Before It Breaks Delivery
- Email Agent Permission Modeling: Map Blast Radius Before Mailbox Access
- How to Red-Team an Email Agent Before It Touches a Real Mailbox
- Email Evidence Hashing: Build a Technical Manifest Without Losing the Original
- Exchange Online NDR Decision Tree: Route the Error to the Right Evidence
Providers
Error codes
- 550 5.7.26 · gmail
- 550 5.7.1 · gmail
- 550 5.7.515 · outlook
- 550 5.7.703 · exchange-online
- 550 5.7.705 · exchange-online
- 550 5.7.708 · exchange-online
- 550 5.7.750 · exchange-online
- 550 5.1.8 · exchange-online
- 550 5.4.1 · exchange-online
- 421.temporary · yahoo
- 553.554.permanent · yahoo
- 421 4.7.28 · gmail
- 550 5.7.23 · exchange-online
- 550 5.7.25 · exchange-online
- 550 5.7.57 · exchange-online
- 550 5.7.64 · exchange-online
- 550 5.7.321 · exchange-online
- 550 5.7.322 · exchange-online
- 550 5.7.367 · exchange-online
- 550 5.7.509 · exchange-online
- 550 5.4.14 · exchange-online
- 550 5.4.8 · exchange-online
- 550 5.1.10 · exchange-online
- 550 5.1.20 · exchange-online
- 550 5.7.506 · exchange-online
- spf.permerror · generic
- dmarc.fail · generic
Mail Failure Doctor reads the complete rejection — status code, enhanced code and provider text — and turns it into a prioritized fix plan.
Diagnose now