How to isolate failures by domain or ISP?

• When deliverability drops, isolating problems to specific domains or ISPs focuses your troubleshooting.
• Domain extraction:
• Parse recipient addresses to extract domains (everything after @)
• Group related domains (gmail.com and googlemail.com are both Google)
• Identify major ISPs vs corporate/other domains
• Analysis steps:

1. Calculate delivery rate per domain: delivered / (delivered + bounced + failed)
2. Compare against your overall rate and historical rates
3. Identify domains significantly below average
4. Check the specific error messages for problem domains

• Common patterns:
• One major ISP failing: Likely reputation or blocklist issue at that provider
• All domains failing: Problem is likely your sending IP or authentication
• Small domains failing, big ISPs fine: Possibly DNS or routing issues to smaller servers
• Specific error across domains: Content or authentication problem
• Tools to use:
• ESP domain-level reporting
• Export logs and analyze with spreadsheet pivot tables
• Google Postmaster Tools (for Gmail specifically)
• Microsoft SNDS (for Microsoft domains)

Domain isolation often reveals that a scary-looking overall failure rate is actually one provider's problem, which is much easier to address.