The ESP Alternative for AI Agents
Traditional email service providers were built for humans clicking through dashboards. If your agent needs to generate variants, launch a subject line A/B test, and read results on its own, marginal is an ESP alternative for AI agents that speaks the protocol they already use: MCP.
Why agents need a different kind of ESP
Most ESPs expose REST APIs designed around campaigns, lists, and templates that a person assembles by hand. An AI agent has to reverse-engineer that UI-shaped surface area to do anything useful, and it still can't reason over the experiment loop end to end.
marginal is an email MCP server instead. It presents a small set of typed tools an agent can call directly, so AI agent email marketing becomes a closed loop: draft, test, measure, iterate — without a growth marketer translating intent into a dashboard.
- No template builder or campaign UI to navigate
- Tools map to intent, not to screens
- Open and click tracking returned as structured data
- Within-test lift metrics the agent can act on
What marginal gives your agent
Connect once at https://marginal.sh/mcp and your agent gets four tools that cover the whole subject line A/B test workflow.
- generate_variants — produce candidate subject lines to test
- launch_test — start a managed send with an A/B split
- get_results — pull opens, clicks, and within-test lift
- recommend_next — get a data-backed suggestion for the next iteration
Works with the tools you already run
Because marginal is hosted and speaks MCP, it drops into any MCP-capable client without you standing up infrastructure. There is no self-hosted server to operate and no separate ESP account to wire credentials through — you authenticate with a Bearer API key (marg_live_...) and start sending.
That makes it a practical ESP alternative for AI agents living inside the editor or runtime where your team already works.
- Cursor, Claude Desktop, Claude Code, and Windsurf
- OpenAI Codex, Cline, Continue, and Zed
- Endpoint: https://marginal.sh/mcp · Registry: sh.marginal/mcp
Start testing on the free tier
You can run real subject line experiments before committing anything. The free tier includes 100 experiments per month, which is plenty to validate whether agent-driven email marketing fits your workflow.
When you're ready to go deeper, the docs at https://marginal.sh/docs/ walk through each tool, the expected payloads, and how lift is calculated within a test.
- 100 experiments/month at no cost
- Managed sends with open/click tracking included
- Full tool reference at https://marginal.sh/docs/
Get started with marginal
marginal is a hosted email marketing MCP server at marginal.sh. Sign up free, create an API key, and connect https://marginal.sh/mcp from Cursor, Codex, or Claude Desktop.
- 100 experiments/month on the free tier
- Four MCP tools: generate_variants, launch_test, get_results, recommend_next
- Listed in the MCP Registry as sh.marginal/mcp