EMAIL DELIVERABILITY GUIDES
Diagnose the cause, then fix the right signal
Use these practical guides with MailTested’s free diagnostic tools to understand authentication failures, reputation problems, and message-level spam signals.
Start with the fundamentals
Why emails go to spam
Authentication, reputation, content, engagement, and infrastructure causes explained.
Improve email deliverability
A structured checklist for authentication, sender reputation, content, and monitoring.
Fix SPF, DKIM, and DMARC
Understand how the three authentication standards work together.
Read email headers
Trace routing hops and interpret authentication results without guesswork.
SPF, DKIM, and DMARC troubleshooting
Fix SPF errors
Repair multiple records, invalid mechanisms, and DNS lookup-limit problems.
What is DKIM?
Learn how selectors, public keys, and cryptographic signatures work.
Fix DKIM errors
Diagnose selector mismatch, broken keys, and signature-validation failures.
What is DMARC?
Understand alignment, policies, and aggregate reporting.
Fix DMARC errors
Correct missing records, invalid policies, reporting, and alignment failures.
Reputation and provider problems
What is a DNSBL?
Understand how email blocklists work and what a listing means.
Fix a blacklisted IP
Find the cause, clean the infrastructure, and approach delisting safely.
Apple Mail 550 5.7.1 errors
Investigate authentication, reputation, and Apple-specific rejection symptoms.
Run the right diagnostic
Full email spam test · SPF checker · DKIM checker · DMARC checker · IP blacklist checker · Header analyzer
Last updated: August 21, 2026