Free Tier Email Experimentation
If you want to test subject lines without committing budget upfront, marginal's free tier gives you 100 experiments per month through a hosted email MCP server. It's enough to validate the workflow, wire it into your AI agent, and start measuring real open and click lift before you scale.
What the free tier covers
Free tier email experimentation on marginal means you get a working subject line A/B test pipeline without a paid plan. Each experiment counts against a monthly quota of 100, which resets on a rolling cycle, so you can iterate on real campaigns rather than toy examples.
- 100 experiments per month at no cost
- Generate subject line variants, launch tests, and pull results
- Open and click tracking with within-test lift metrics
- Managed sends — marginal handles delivery, you handle strategy
How AI agent email marketing fits in
marginal is built as an email MCP server, which means your AI agent talks to it over the Model Context Protocol instead of a custom HTTP integration. An agent in Cursor, Claude Code, or Continue can call the available tools directly during a conversation, turning a vague 'test these subject lines' request into a launched experiment.
This is what makes AI agent email marketing practical on a free tier: the agent does the orchestration, and you only spend experiments when you actually launch a test.
- generate_variants — draft subject line options for a campaign
- launch_test — start a subject line A/B test with managed sends
- get_results — fetch open/click metrics and lift
- recommend_next — get a data-backed suggestion for the next iteration
Running your first subject line A/B test
A typical loop on the free tier looks like this: ask your agent to generate a handful of subject line variants, launch a test against a segment, wait for opens and clicks to accumulate, then read the within-test lift to see which line won. Because each step is an MCP tool call, the whole thing happens inside your editor or agent session.
Point your client at the endpoint https://marginal.sh/mcp and authenticate with a Bearer API key (marg_live_...). The setup guide at https://marginal.sh/mcp walks through connecting Claude Desktop, Windsurf, Zed, Cline, and OpenAI Codex.
- Endpoint: https://marginal.sh/mcp
- Registry: sh.marginal/mcp
- Auth: Bearer API key
- Docs: https://marginal.sh/docs/
When to move beyond free
The free tier is sized for experimentation, not high-volume production. If you're consistently bumping against 100 experiments a month, that's a sign your subject line testing has become part of your regular campaign cadence — and a paid plan removes the cap. Until then, the free tier is a low-risk way to prove the email MCP workflow works for your team.
- Stay on free while you're validating the approach
- Watch your monthly experiment count as a usage signal
- Upgrade when testing becomes routine rather than experimental
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