Campaign Monitor

The API covers subscribers, lists, campaigns, transactional email, and client account management. The client account management capabilities make Campaign Monitor particularly useful for agents in agency contexts where a single API key can manage email programs across multiple client brands and lists. Subscriber management — adding, updating, and segmenting subscribers — is well-supported for automated list maintenance workflows.

Campaign Monitor is an established email marketing platform with a clean, consistent API used by agencies and mid-market businesses. For agents handling email marketing automation — managing subscriber lists, triggering campaigns, handling transactional email through their Transactional API — Campaign Monitor provides reliable infrastructure with an API that reflects years of developer-focused improvement. The agency model (managing multiple client accounts) gives it a different usage pattern from consumer-focused email platforms.

Authentication uses API keys at both the user and client level. The two-level model enables agents to operate at the right scope — user-level keys for global account management, client-level keys for scoped access to individual client programs. Teams should use the narrowest scope appropriate for each agent integration.

Documentation is complete and well-organized. The API reference covers all available endpoints with examples in multiple languages. Campaign Monitor's documentation quality reflects the product's developer audience. Teams integrating email marketing automation will find the docs sufficient for production implementation.

Reliability is appropriate for an established email marketing platform. Campaign sending is asynchronous — triggering a campaign via API schedules it, and delivery occurs over time. Agents that need to track campaign delivery should use the reporting API endpoints to monitor sending progress rather than expecting synchronous delivery confirmation from the send trigger.