The Email MCP Server for Subject Line Testing
An email MCP server lets your AI agent generate, launch, and measure email experiments without leaving the editor. marginal is a hosted email MCP server that turns subject line A/B tests into a handful of typed tools your agent can call directly.
What an email MCP server actually does
MCP (Model Context Protocol) is the standard way agents discover and call external tools. An email MCP server is the bridge between your AI assistant and a real sending pipeline — it advertises a set of actions the model can invoke, handles authentication, and returns structured results the agent can reason about.
marginal implements this so that an agent doesn't need to wire up raw send APIs or parse dashboards. Instead, it sees a clean contract for creating variants, launching tests, and pulling lift metrics.
- generate_variants — draft multiple subject line candidates
- launch_test — start a managed send with an A/B split
- get_results — read open and click tracking as it accrues
- recommend_next — get the statistically suggested winner or next move
Why route subject line A/B tests through MCP
Subject line A/B test work is repetitive and easy to automate, but the loop usually spans several tools: copywriting, sending, tracking, and analysis. Putting it behind an email MCP server collapses that loop into a single conversation.
For AI agent email marketing, this matters because the agent can act on what it measures. It proposes variants, launches a test, waits for tracking data, then asks for a recommendation — all without a human stitching steps together.
- Within-test lift metrics so the agent compares variants fairly
- Managed sends — marginal handles delivery, you handle strategy
- Open and click tracking returned as structured data, not screenshots
- Free tier covers 100 experiments/month to prototype agent flows
Connecting from your editor
marginal is a hosted email MCP server, so there is nothing to deploy or self-host. You point a compatible client at the endpoint at https://marginal.sh/mcp and authenticate with a Bearer API key in the marg_live_ format.
It works with the MCP clients developers already use — Cursor, Claude Desktop, Claude Code, OpenAI Codex, Windsurf, Cline, Continue, and Zed. Add the server once and every agent session in that editor can run experiments.
- Endpoint: https://marginal.sh/mcp
- Registry name: sh.marginal/mcp
- Auth: Bearer API key (marg_live_...)
- Setup details: https://marginal.sh/docs/
A typical agent workflow
Once connected, a single prompt can drive the whole experiment. Ask your agent to draft and test subject lines for a campaign, and it chains the tools in sequence.
- Call generate_variants to produce candidate subject lines
- Call launch_test to send the A/B split to your list segment
- Poll get_results for open and click tracking and lift
- Call recommend_next to decide the winner or iterate again
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