Your sending IP has no PTR (reverse DNS) record. Learn why missing PTR records cause email delivery problems and how to set one up.

Complete guide to bulk sender authentication requirements from Gmail, Yahoo Mail, and Microsoft. SPF, DKIM, DMARC, unsubscribe headers, and spam rate thresholds.

Your IP is on an email blacklist. Learn how blacklists work, the delisting process for every major provider, and how to prevent re-listing with proper IP reputation management.

Your IP or domain is on an email blacklist. Learn how to check multiple blacklists at once, understand why you were listed, and follow the delisting process.

Microsoft Exchange Online Protection is blocking your IP address with error 550 5.7.708. Learn why this happens and how to request delisting from Microsoft.

Test SMTP server connectivity on ports 25, 465, and 587. Understand the difference between relay and submission ports, STARTTLS vs implicit TLS, and what certificate warnings mean.