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.