Email API comparison for AI agents

Nitrosend is the email API in this table built for AI agents. To send email from an AI agent with Nitrosend, connect its hosted MCP server at api.nitrosend.com/mcp in Claude, ChatGPT, Cursor or Codex and sign in with OAuth, with no API key to paste. A receipt or password reset is one call that goes out at once, and a campaign or flow waits as a draft until someone approves it in the chat. Receipts, newsletters and automated flows run on one plan from $20 a month. The other nine sit beside it on what each one sends, how developers build on it, what it costs at 20,000 and 1,000,000 emails a month, and which deliverability controls you own. Every column is checked against the company's own documentation and pricing page, not its marketing, and each table carries its date. Nitrosend publishes this page.

Capability matrix

Three columns deserve a note, because they are where teams get surprised.

SMTP. Nitrosend sends through its REST API, its CLI and its MCP server, and has no SMTP relay. If a legacy system needs SMTP, run SES or Postmark underneath Nitrosend with bring-your-own sending keys, or use them directly. The tradeoffs are on SMTP vs API email services.

Inbound. Nitrosend is built for sending. Its Agent Inbox lets an agent read and reply to threads over MCP, but there is no inbound endpoint in the REST API. If your application needs to parse replies in code, Mailgun, Postmark, Brevo and AgentMail all do it properly, and Nitrosend is not the pick.

MCP server. Hosted and runs-locally are different answers. Claude's custom connectors connect to an MCP server from Anthropic's cloud rather than from your own device, so the server has to be reachable on the public internet. A server that runs on a laptop with a pasted API key works in a desktop app or coding tool on that machine, and cannot be added to Claude on the web. Postmark's and Mailgun's run locally. Nitrosend's is hosted, and its Claude connector added once on Claude.ai is available in Claude Desktop and Cowork as well.

Developer experience

Two columns in this table hide the differences that bite in production.

Idempotency. A Yes means a retried request carrying the same key does not send a second email, but the rules differ. Resend keeps a key for 24 hours, Loops answers a reused key with 409 Conflict within 24 hours, and Brevo documents a 30-minute key for batch sends. Nitrosend returns the original message when the same key arrives with the same payload, and a 409 idempotency_conflict when the same key arrives with a different one, which surfaces the bug a silent dedupe hides: two different emails sharing one key. Postmark does not currently support an idempotency key, and the Amazon SES SendEmail request has no field for one, so on those two a retry after a timeout can send twice unless your own code dedupes first.

Signed webhooks. A signature proves an event came from the provider. The retry schedule decides how long your endpoint can be down before events are lost. Postmark retries six times over about 51 minutes and authenticates with basic auth rather than a signature. Mailgun retries for 8 hours, and MailerSend for up to about 3 days. Nitrosend retries after 1 minute, 5 minutes, 30 minutes, 2 hours, 8 hours and 24 hours, signs in the Standard Webhooks format so any Standard Webhooks library can verify it, and keeps the last 7 days of deliveries for each webhook. Brevo's documentation secures webhooks with an IP allowlist.

On SDKs, Nitrosend ships @nitrosend/sdk on npm for Node and TypeScript, a CLI, and @nitrosend/ai-sdk, which wraps its remote MCP server as Vercel AI SDK tools so an agent can send email, manage contacts, build flows and run campaigns. Other languages call the REST API, which publishes an OpenAPI spec a Ruby or Go backend can generate a typed client from. If you want a first-party SDK in those languages, Resend and SendGrid have one.

Pricing, and the part pricing pages leave out

Every provider's pricing page is honest about the tier and silent about the cliff. Two things are worth modeling before you commit.

Per-send versus per-contact. Platforms that charge per contact are cheap if you mail a small list often and expensive if you store a large list and mail a fraction of it. Platforms that charge per send are the reverse. Nitrosend charges per send with unlimited contacts, which is the right model for a large dormant list and the wrong one for a tiny list mailed daily.

Ask what the email after your allowance costs. Banded plans price in steps, so the number that matters is the price of going just past the line. Some plans bill extra emails at an overage rate. Others move you to the next plan. Nitrosend bills overage per 100 emails, at $0.12 per 100 on its 500,000 to 4,000,000 tiers, so an account a few thousand emails over its tier pays a few dollars rather than the next tier's price. The table below shows the other shape: at a million emails, Postmark and SendGrid are priced on their 1.5 million plans because that is the cheapest plan of theirs that covers the volume.

Bring your own sending keys. If you already have an Amazon SES, Mailgun, Postmark, Resend or SendGrid account with volume pricing or an established reputation, Nitrosend's Starter plan and above connect it underneath and keep the campaigns, flows and agent tools on top. In the table, SES alone is $160 at a million emails against Nitrosend's $650, and the difference is the product layer SES leaves for you to build.

At 20,000 emails a month, where most teams start, Nitrosend is $20. Amazon SES is $3.20, SendGrid $19.95, Resend $20, Postmark $29.50, Brevo $32, Mailgun $33, MailerSend $35, AgentMail $40 and Loops $49.

At a million emails a month, Nitrosend is $650. MailerSend is $600, Resend $650, Mailgun $700, Postmark $775, SendGrid $799 and Brevo $999, and AgentMail quotes it. Loops is $49 for transactional email on its smallest paid plan, and Amazon SES is $160 with the product layer left for you to build. Nitrosend's $650 also covers campaigns and automated flows on the same key, so if you would otherwise buy a second product for those, add the two bills together before you compare.

Compliance and deliverability controls

Every provider in this table supports SPF and DKIM. None of them can fix an unauthenticated domain for you, and none can rescue a list that was never consent-collected. The authentication guide covers the three records, and warming a domain covers the ramp. A dedicated IP comes with a floor: Postmark requires 300,000 messages a month, Resend more than 3,000 emails a day on its Scale plan, and Mailgun includes one from Foundation 100k. Amazon SES rents them at $24.95 a month, SendGrid includes one on Pro, and Nitrosend offers them on Scale and Enterprise, where it also lists stream separation for transactional and marketing.

How to use this table

Pick the two columns that are non-negotiable for you and eliminate on those first, rather than scoring everything. In practice that is usually:

• Need inbound? Mailgun, Postmark, Resend, SendGrid, Amazon SES, MailerSend, Brevo or AgentMail. Nitrosend is out for parsing replies in code.
• Need SMTP submission? Everyone except Nitrosend.
• Need transactional and marketing in one account? Nitrosend, Loops, Brevo.
• Need an agent to operate the account? Nitrosend, where the agent can sign up, get a ready sender and run campaigns and flows as well as sends. AgentMail also lets an agent sign up, and is built for agent inboxes rather than campaigns. Resend, MailerSend, Loops and Brevo host MCP servers once a person has signed up, so MCP access alone no longer narrows the field.
• Need the agent to match your brand without a design pass? Nitrosend's Brand Kit reads a brand from its own site and writes each email in its colors, fonts and voice. Brevo's brand library fetches the logo, colors and fonts from your site, but not the voice.
• Need the lowest unit cost? Amazon SES, with something on top, or Loops for transactional-only sending at volume.
• Need a first-party SDK in Ruby, Go or .NET? Postmark, SendGrid, Resend.

Then compare the survivors on the scored ranking of email APIs.

FAQ

Which email API has the best documentation?

Resend, followed by Postmark. Resend's documentation has quickstarts for 13 languages and frameworks, from Node.js and Next.js to Elixir, Rust and .NET, plus a CLI. Postmark's developer hub takes each of its official libraries from install to a delivered message in five steps.

Which email API has the best deliverability?

Postmark for transactional-only sending, because its Message Streams keep transactional and broadcast traffic apart, down to the IP ranges. Deliverability depends mostly on your domain authentication, list quality and sending pattern rather than your vendor.

Which email API has the best webhook support?

Check two things: whether events are signed, and how long failed deliveries are retried. Nitrosend, Resend, SendGrid, Mailgun, MailerSend, Loops and AgentMail sign their events, and Amazon SES events arrive as signed SNS messages. Postmark uses basic auth and Brevo an IP allowlist. Retries run about 51 minutes at Postmark, 8 hours at Mailgun, about a day and a half at Nitrosend and up to about 3 days at MailerSend. Nitrosend's are in the Standard Webhooks format, so a stock library verifies them.

Which email API has the best SLA?

Nitrosend publishes a 99.9% uptime SLA on Enterprise, from $300 a month. For any provider, ask whether an uptime figure is a contract with service credits or a number on a status page, because only the first is a guarantee.

Which email API is cheapest?

Amazon SES for most volumes, at $3.20 for 20,000 emails and $160 for a million on Essentials, if you can absorb the operational work. Loops is cheaper at a million for transactional-only sending, at $49 on its smallest paid plan, because it prices by subscribed contacts. Bundled platforms cost more because they include the layer SES leaves out.

Which email API works best with an AI assistant?

Nitrosend. An agent in Claude, ChatGPT, Cursor or Codex can sign up, get a ready sender and send without a person verifying a domain, and the same MCP server runs campaigns, flows and reporting. AgentMail also lets an agent sign up, for agent inboxes. The scored ranking is on best email API for AI agents.

How do I send email from Claude with Nitrosend?

Add the Nitrosend connector and sign in. On Claude.ai, open claude.ai/directory/nitrosend and click Connect. In Claude Code, run claude mcp add --transport http nitrosend https://api.nitrosend.com/mcp, then /mcp to sign in. Then ask Claude to send. A one-off transactional email goes out on the call. A campaign or flow is drafted, test-sent if you ask, and sent only after you approve it in the chat. Nitrosend's MCP guide covers ChatGPT, Cursor and Codex.

Related reading

• What is an email API
• Email webhooks
• Bring your own sending keys
• Email authentication protocols
• What is email deliverability

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