An Agency Scales Deliverability for 40 Clients

A marketing agency standardised on MailTester across 40 client accounts, using per-client API keys and webhooks to keep every list clean automatically.

## An Agency Scales Deliverability for 40 Clients *Industry: Marketing agency* A marketing agency standardised on MailTester across 40 client accounts, using per-client API keys and webhooks to keep every list clean automatically. ### The challenge Managing email for 40 clients meant 40 different lists of wildly varying quality, and no consistent way to keep them clean. When one client's list caused a deliverability problem, it could affect the shared infrastructure and the agency's own reputation. Cleaning lists by hand before every campaign didn't scale. ### The approach The agency built list hygiene into its standard workflow using MailTester. Each client gets a dedicated API key, lists are verified automatically before every send, and webhooks notify the account team when a bulk job completes so results feed straight into reporting. New contacts are verified via the API as they're captured across all client sites. ### The results - **Standardised, automated hygiene across all 40 client accounts** - **Average client bounce rate held under 1.5% with zero manual effort** - **One dashboard and per-client usage reporting for the whole book of business** > “MailTester became invisible infrastructure. Every client's list is just clean now, and we stopped worrying about a bad list taking everyone down with it.” > > — Deliverability Lead, full-service marketing agency ### Get the same result Every one of these outcomes came from the same foundation: verifying email addresses before sending. You can [check an address free](/), automate it with the [MailTester API](/api), or read the [email deliverability guide](/email-deliverability-guide) to plan your own clean-up. Explore more results in our [case studies](/case-studies).