How to isolate failures by domain or ISP?

Your deliverability drops. But is it everywhere, or just at Gmail? Just at corporate Outlook domains? Just at Yahoo? The answer changes everything about what you do next, so the first step is always to segment your failures by recipient domain.

Start by pulling your bounce and deferral data and grouping by recipient domain (everything after the @ symbol). Most ESP dashboards have a domain-level breakdown somewhere. If you're working with raw logs, extract the domains with a quick script or spreadsheet pivot. Group related domains together, gmail.com and googlemail.com are both Google, hotmail.com, outlook.com, and live.com are all Microsoft.

Once you have domain-level failure rates, the diagnosis gets much easier. Problems at all major ISPs simultaneously usually mean a global reputation issue, your sending domain or IPs are in trouble across the board, and you need to look at sender reputation data, blocklist status, and authentication.

Problems at one specific provider usually mean something that provider specifically doesn't like. Google Postmaster Tools will tell you your domain reputation at Gmail. Microsoft SNDS covers Outlook. Yahoo has its own postmaster portal. Each shows you complaint data and reputation scores from their specific filtering perspective.

Corporate domains (company email addresses, not consumer ISPs) can be harder to diagnose because they often run their own filtering systems without public postmaster tools. When you see failures clustering at corporate addresses, check whether your authentication is completely clean, since enterprise security gateways are often stricter on that front. Our free Blocklist Checker can help identify if domain-specific blocking is driven by a blocklist listing.