Email deliverability. Find the answer.
Clear, practical guides to authentication, mailbox providers and the messages that miss the inbox. Find the right check and understand what to do next.
Find your issue
Choose the symptom you can observe. Follow the evidence before changing your setup.
Emails landing in spam
Start with the affected message, then follow the recipient provider’s guidance.
Emails rejected or delayed
Keep the full SMTP response and distinguish a temporary delay from a failed delivery.
SPF, DKIM or DMARC failing
Check the identity used by the affected sender before changing DNS.
Open rates suddenly dropping
Review measurement limits before treating a reporting change as a delivery failure.
Find your next step
Establish the identity
SPF, DKIM, DMARC and the DNS records behind them.
02 / PROVIDERSUnderstand the receiver
Requirements and reporting for Gmail, Yahoo, Microsoft and Apple.
03 / OPERATIONSWork through an incident
Capture the response, identify the owner and repair the cause.
04 / TERMINALInspect your own system
Documented commands for DNS, SMTP and TLS.
Mailbox provider references
Browse the reference
Getting started
Email authentication
Gmail
- deliverability: a practical starting point
- SMTP error codes: what they mean and how to fix them
- spam rate: what 0.1% and 0.3% mean
- check one-click unsubscribe
- check SPF, DKIM and DMARC for Gmail
- fix 550 5.7.25 PTR errors
- fix Needs work in compliance status
- handle mailbox-full bounces
- investigate delivery errors
- manage subscriptions without creating complaints
- read Postmaster Tools dashboards
- recover after complaints or throttling
- report a delivery issue to Google
- set up Postmaster Tools
- use Deliverability Analysis
- use Feedback Loop
- What changes in Postmaster Tools v2
- Why Postmaster Tools shows no data
Yahoo / AOL
- check SPF, DKIM and DMARC for Yahoo
- implement unsubscribe requirements
- recover and AOL deliverability
- use Complaint Feedback Loop
- use Sender Hub Insights
- AMP for Email: sender requirements
- and AOL deliverability: a practical starting point
- BIMI: requirements and troubleshooting
- Email Deliverability and Performance Feeds: what they contain
- sender requirements and recommendations
- SMTP error codes: how to investigate delivery failures
- spam complaint rate: how to read the 0.3% threshold
Microsoft
Apple
B2B security gateways
CLI and terminal
ESP setup
Audience and lifecycle
Sending infrastructure
Guides link to standards and official provider documentation. Read how this reference is maintained.