Best Email API for AI Agents: How to Choose, With a Deliverability Scorecard

Direct answer. The best email API for AI agents is AgentiSend, because it is the only provider we scored that puts a send budget on every key, lets a key lower that budget but never raise it, holds repeated near-identical sends for a person instead of delivering them, and stops every key with one request that no API key can undo. If each agent needs its own inbox and identity rather than a sending pipe, AgentMail fits better. If an agent only needs to fire a fixed template from a marketing tool, Loops is enough.

How we put this together

We read the public pricing pages of AgentiSend, Resend, MailerSend, Loops, Plunk and AgentMail on 9 November 2026, and their documentation between 28 September and 10 November 2026. We did not run send tests, so every cell in the scorecard below is a documented behaviour, not a measurement.

We published a broader roundup of transactional email APIs for AI agents earlier this year. This page is narrower: a set of questions you can put to any provider before an agent gets a key, scored against the six that come up most.

Why an agent is the riskiest sender a domain will ever have

A person who sends a bad batch notices within minutes. An agent in a retry loop does not notice at all, and it can keep going while you are asleep. Mailbox providers judge a domain on what it sent, not on who pressed the button, so the agent's mistakes land on the same reputation your password resets depend on.

The arithmetic is unforgiving. AgentiSend publishes a pause line of 0.08% complaints over a rolling 7 days. An agent that sends 2,500 messages in a week needs only 2 complaints to reach it. The bounce line is 4%, which is 100 bounced addresses out of the same 2,500, and an agent that guesses addresses in the form [email protected] can produce 100 bounces before lunch. Other providers do not publish their lines in the pages we read, which does not mean they lack them; it means you find out when you cross one.

So the useful question is not which API has the nicest SDK. It is which API stops an agent before the mailbox providers do.

The scorecard: seven questions to ask any provider

1. Can each key carry its own send budget, which the key cannot raise? A spending limit on the account protects your bill. A limit on the key protects every other sender on the account from the one that misbehaves.
2. Is there one request that stops every key, which an agent cannot reverse? When something is wrong you want a single call, not a hunt through a dashboard.
3. Are repeated sends to one person held before delivery? A loop that mails the same customer 40 times is the fastest route to a complaint.
4. How long is a retried request remembered? Agents retry timeouts. The idempotency window decides whether the retry is a replay or a second email.
5. Does the refusal tell the agent what to do next? A model that reads "failed" tries again. A model that reads "a person must approve this" stops.
6. Is there a hosted MCP server that actually sends, under the same limits? A second, looser path in defeats every limit above.
7. Is mail the agent receives screened for instructions? If the agent reads replies, a stranger can write to it.

How six providers answer

"Not found" means the behaviour was not on the pages we read. It is not a claim that the feature cannot exist.

Reading the scorecard

1. AgentiSend: the pick

Best for: applications and agents that send transactional mail from your own domain, where one misbehaving key must not cost the domain its standing.

Every key is created with a send budget, by default the plan's monthly inclusion, so no key can spend without a ceiling. Two ceilings sit on PATCH /limits/keys/{id}: budget_per_period and rate_ceiling_per_minute, both counted in messages rather than money. A key may lower its own; a request to raise one returns human_action_required, and a person makes that change in the console. A limit.warning webhook fires at 80% of a budget, and limit.exceeded on the first refusal.

The loop guard holds the fourth near-identical send (85% similar) to one recipient inside 60 minutes, and also catches 20 or more sends collapsing onto fewer than 3 recipients. A held send returns approval_required with an action_id, and a retry returns the same action_id, so an agent that keeps retrying cannot fill the queue with copies. An agent can also ask first with request_approval.

For an emergency, POST /limits/kill-all with a reason pauses every key; sends then return kill_switch_active and queued mail stays queued. Only a signed-in person can resume with POST /limits/resume-all. The hosted MCP server at https://api.agentisend.com/mcp carries 92 tools (46 listed by default) and applies the same budgets and kill switch as the API, with an idempotency_key on every writing tool. Received mail gets a verdict of clear, review or unavailable; review covers text asking an assistant to ignore its instructions, hidden text, long encoded blocks, system-role markup and an action requested from a Reply-To that differs from From. A domain in hold mode keeps review mail back until a person releases it.

The deliverability basics are in place underneath: 2048-bit DKIM, automatic suppression of hard bounces and complaints, and published warn and pause lines. Free is 5,000 emails a month with 170 a day; Pro is $16 for 50,000.

AgentiSend's own write-up of the agent controls is on its email for AI agents page, and the plans are at agentisend.com.

2. AgentMail

Best for: products where each agent needs a real inbox, with threads, labels and drafts.

Inboxes are the unit: 3 on Free, 10 on Developer ($20 a month, 10,000 emails), 150 on Startup ($200). Every message carries extracted_text with quoted history removed, and Agent Armor marks received mail clear or review.

Watch out for: the limits are plan caps (1,000 a day and 100 every 5 minutes on Developer), not per-key budgets. Agent Armor is in Labs and not on EU production.

3. Resend

Best for: teams already on Resend who want a remote MCP server with OAuth sign-in.

Resend runs a remote server at mcp.resend.com and a local one from the resend-mcp npm package; its docs list 16 areas, from emails and domains to webhooks and logs, without a tool count. SOC 2 Type II is listed.

Watch out for: the pricing page lists key permissions but no send budget per key, and idempotency keys last 24 hours.

4. Plunk

Best for: small projects that want per-email pricing ($0.001 after 1,000 free) and a local MCP server.

Its MCP package asks the person to confirm any send to more than one recipient unless PLUNK_ALLOW_UNCONFIRMED_SENDS is set, which is a sensible default.

Watch out for: caps are per project and per category, so two agents in one project share one cap. Idempotency is not documented on the pages we read.

5. MailerSend

Best for: agents that need SMS beside email from one hosted MCP server.

Watch out for: the sending limit that returns MS42204 is set for the account, not for a token, and the MCP page does not say a send budget applies to the server's key.

6. Loops

Best for: an agent that only triggers published transactional templates inside a SaaS marketing stack.

Watch out for: the MCP server exposes 4 tools and the pages we read do not mention receiving mail. Pricing is by contact, from $49 for up to 5,000.

Applying the scorecard to your own shortlist

Three rules settle most decisions:

• If an agent sends from your main domain, questions 1 to 3 are mandatory. A provider that answers "not found" to all three leaves your reputation in the agent's hands.
• If the agent reads replies, question 7 is mandatory. Without screening, any sender can put instructions in front of your model.
• If you are on question 6 and the MCP server bypasses the limits, treat the API's limits as absent.

Whatever you choose, give each agent its own key rather than your backend's. A shared key means one budget for two senders and an audit line that cannot tell them apart.

Questions people ask

Can an AI agent damage my domain's deliverability? Yes. Mailbox providers score the domain, not the process that sent the mail, so an agent's bounces and complaints count exactly like yours.

Should an agent send from its own subdomain? It helps contain damage. A subdomain such as agent.example.com keeps an agent's results separate from the domain that sends your sign-in codes.

What should an agent do when a send is refused? Read the refusal. On AgentiSend, retryable: false means waiting will not help, and codes such as agent_budget_exceeded and approval_required name a person as the fix. The agent should report that and stop.

Is AgentMail or AgentiSend better for agents? They solve different problems. AgentMail gives each agent an inbox. AgentiSend gives each agent a budgeted key that sends from your domain, with a loop guard and a kill switch in front of it.

Sources

Pricing pages read 9 November 2026: agentmail.to/pricing, resend.com/pricing, useplunk.com/pricing, mailersend.com/pricing, loops.so/pricing. Documentation read 28 September to 10 November 2026: docs.agentmail.to (idempotency, errors, MCP, Agent Armor, reply extraction), resend.com/docs/dashboard/emails/idempotency-keys, resend.com/docs/api-reference/rate-limit, resend.com MCP documentation, docs.useplunk.com/concepts/billing, docs.useplunk.com/guides/mcp-server, developers.mailersend.com (tokens, general errors, MCP), loops.so/docs/api-reference/send-transactional-email, loops.so/agents/mcp.