Choose who sends your email

Declare the providers on the process that sends

Application configuration has two jobs: enable EMAIL_TRANSACTIONAL in engineCapabilities, then make its providers available in the backend scope. Selection chooses among that declared set; it cannot install an undeclared provider.

Application example, reduced from Wonder CRM’s wildo.saas.config.ts to the complete email-provider section. Assign emailProviders to the existing configuration’s providers property, merging the other provider scopes your application uses.

import { EngineCapability } from '@wildo-ai/saas-models';
import { defineSaaSProviders } from '@wildo-ai/platform-config-lib';

const emailProviders = defineSaaSProviders({
  scopes: {
    backend: {
      providers: {
        resend: {
          engineCapabilities: [EngineCapability.EMAIL_TRANSACTIONAL],
          providerCapabilities: ['EMAIL_TRANSACTIONAL'],
          protocols: ['EMAIL_PROVIDER'],
        },
        'email-preview': {
          engineCapabilities: [EngineCapability.EMAIL_TRANSACTIONAL],
          providerCapabilities: ['EMAIL_TRANSACTIONAL'],
          protocols: ['EMAIL_PROVIDER'],
        },
      },
      selection: {
        [EngineCapability.EMAIL_TRANSACTIONAL]: { primary: 'resend' },
      },
    },
  },
});

Keep [EngineCapability.EMAIL_TRANSACTIONAL]: { enabled: true } in the application’s engineCapabilities. The declaration makes both providers reachable; the selection makes Resend the default. Backend saas-config.backend.ts supplies email.from, and the existing backend module supplies emailTemplateDefinitions.

Change only the local selection

Wonder CRM’s infrastructure/local/wildo.infra.local.config.ts contains this property inside its infrastructure configuration:

providerSelection: {
  [EngineCapability.EMAIL_TRANSACTIONAL]: {
    primary: 'email-preview',
  },
},

Local messages now render into logs; environments without that override retain Resend. Supply RESEND_API_KEY through the sending environment’s secret configuration and use a sender accepted by that provider. Keep credentials out of templates and tracked configuration.

From the application root, wildo config sync --env local regenerates the local configuration and environment artifacts. Inspect the generated backend provider runtime for both declared references and check the effective local selection. A send through the local flow should return LOCAL_PREVIEW; a network provider reports its own classified submission result.

Selection is not retry failover

Email resolves one provider for a send. Availability selection and an environment override do not mean a failed network submission is automatically retried with another vendor. Interpret the sender’s outcome and retryability: an uncertain response may already represent an accepted message.