Start with the foundation: DNS authentication
Most deliverability symptoms (junk folder placement, low reputation scores, hard bounces with no response) trace back upstream to an authentication gap. Before chasing provider-specific issues, run the free validator to confirm SPF, DKIM, and DMARC are set up correctly.
Validate your SPF, DKIM, and DMARC free
Instantly runs the DNS lookups against your domain and flags any missing or misconfigured record.
Run the free validator →All deliverability guides
Why cold email lands in spam and how to fix it end to end — SPF, DKIM, and DMARC, warm-up, list quality, content, and the seed tests to confirm inbox placement before scaling.
Read guide →The 2024 Gmail + Yahoo bulk-sender checklist every high-volume sender has to clear: SPF, DKIM, DMARC, spam rate under 0.1%, one-click unsubscribe, PTR, and From: alignment.
Read guide →Why legitimate cold email lands in Outlook's Junk folder, Safe Senders across Outlook desktop, OWA, and Microsoft 365, and why JMRP enrollment matters for Outlook.com.
Read guide →Enroll in Google Postmaster Tools, read your domain and IP reputation, fix a "Low" score, and tie it back to your SPF, DKIM, and DMARC stack.
Read guide →Enroll in Microsoft SNDS and JMRP, read your Outlook.com IP reputation and complaint data, and connect it back to your SPF/DKIM/DMARC stack.
Read guide →Every common 4xx and 5xx SMTP response — hard vs soft bounce, what to retry, and how to clean the list fast after a campaign goes out.
Read guide →Also check: are you on a blocklist?
Providers deliverability symptoms often start with a blocklisting — Spamhaus, Cisco Talos, Barracuda, SenderScore, or SORBS is where reputation trouble shows up first.
Check a sender domain or IP against major blocklists →