A no-SMTP email A/B test platform for AI agents
If you want to test subject lines without standing up a mail server, marginal handles sends, tracking, and lift metrics behind a single MCP endpoint — no SMTP configuration required.
Why skip SMTP entirely?
Most teams reach for an email A/B test platform expecting to wire up SMTP credentials, warm an IP, and configure bounce handling before they can send a single variant. That's a lot of plumbing just to learn which subject line wins.
marginal is a hosted email MCP server that takes care of delivery for you. There's no self-hosted server to deploy and no SMTP relay to configure — your agent calls a tool, and the send happens on managed infrastructure.
- No SMTP host, port, or relay credentials to manage
- Managed sends with built-in open and click tracking
- Within-test lift metrics computed for you
- Free tier covers 100 experiments per month
How the subject line A/B test runs
A subject line A/B test on marginal is a sequence of four MCP tools. You generate candidate variants, launch the test against your audience, pull results as opens and clicks accumulate, and ask for a recommendation on what to send next.
Because everything runs through the MCP protocol, an AI agent can orchestrate the whole loop without any glue code or SMTP setup on your side.
- generate_variants — draft subject line candidates
- launch_test — split your list and send through managed delivery
- get_results — open rates, click rates, and within-test lift
- recommend_next — pick the winner and suggest the follow-up
Built for AI agent email marketing
marginal connects to the MCP clients developers already use, so AI agent email marketing fits into your existing editor or assistant. Point a client at the endpoint, authenticate with a Bearer API key, and your agent can run experiments directly.
Supported clients include Cursor, Claude Desktop, Claude Code, OpenAI Codex, Windsurf, Cline, Continue, and Zed.
- Endpoint: https://marginal.sh/mcp
- Auth via Bearer API key (marg_live_...)
- Registry identifier: sh.marginal/mcp
- Setup guides at https://marginal.sh/docs/
What you get without the infrastructure
Choosing a no-SMTP email A/B test platform means you focus on the experiment, not the mail server. marginal measures lift inside each test so your agent can act on real numbers rather than guesses.
Start with the free tier, run subject line A/B tests across your campaigns, and let recommend_next guide your next message — all through the email MCP server at https://marginal.sh/mcp.
- Subject line A/B tests with statistical lift per variant
- Open and click tracking included in managed sends
- Agent-driven workflow from draft to winner
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