# Groundmail API + MCP vs MailPace

> Groundmail scores 64.5 (B) to MailPace's 58.4 (C) for sending email. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/groundmail-vs-mailpace
- Markdown: https://www.anchorterminal.com/compare/groundmail-vs-mailpace.md (~4,550 tokens)
- Site index for agents: https://www.anchorterminal.com/llms.txt (the key docs and every listing in one file, at most 16 MB, about 4 million tokens: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- MCP: https://www.anchorterminal.com/mcp (the whole catalogue as tools, no key)
- DNS: dig +short TXT <slug>.grade.anchorterminal.com (the grade as a DNS TXT record, for a sandbox with no HTTP where names still resolve; also top., best.<category>. and <a>.vs.<b>. under the zone, and the same under grade.letme.dev)
- Licence: CC BY 4.0, credit Anchor Terminal and link this page (https://www.anchorterminal.com/terms/#data)
- Updated: 2026-11-08

Groundmail API + MCP scores 64.5 (B) on agent readiness against MailPace's 58.4 (C), and leads in 4 of 7 scored categories. Both do sending email.

- Groundmail API + MCP: grade B, 64.5/100, rank #462 of 1513. Markdown https://www.anchorterminal.com/tools/groundmail.md · JSON https://www.anchorterminal.com/api/v1/tools/groundmail.json
- MailPace: grade C, 58.4/100, rank #754 of 1513. Markdown https://www.anchorterminal.com/tools/mailpace.md · JSON https://www.anchorterminal.com/api/v1/tools/mailpace.json
- Best email delivery APIs for AI agents: https://www.anchorterminal.com/best/email/index.md
- All 174 email comparisons: https://www.anchorterminal.com/compare/email/index.md

## Which one, for what

### Groundmail API + MCP (B)

Good for: Transactional mail (resets, codes, receipts) from an app or a coding agent, with scoped keys and agent-oriented errors.

Ahead on:
- Schema & documentation, 88 against 54
- Security & auth, 63 against 50
- Payments & pricing, 35 against 25
- Maintenance & community, 77 against 65

Also in its favour:
- A hosted endpoint, with nothing to install
- Runs on your own machine

Watch for: 51 MCP tools with no toolsets, about 114 KB of definitions and output schemas

### MailPace (C)

Good for: Low-volume transactional mail from EU-hosted infrastructure with idempotent sends and no tracking.

Also in its favour:
- No incidents deducted, where Groundmail API + MCP loses 3 points for them

Watch for: No OpenAPI spec, llms.txt or Markdown docs for agents; the reference is Docusaurus pages


## Why the edge

### Schema & documentation: Groundmail API + MCP +34

- Groundmail API + MCP (88): OpenAPI 3.1 at docs.groundmail.dev/openapi.yaml with 70 operations, each naming its scope in `x-required-scope`, and typed input and output schemas on every MCP tool (25, OpenAPI per the 17 October check; a permission check refused the request on 4 November). llms.txt, llms-full.txt, Agent Skills and, since 22 October, a Markdown version of every docs page (10). MCP descriptions run from one line to a paragraph, with a few 'do not combine' rules, and only 21 of 70 OpenAPI operations carried a description on 17 October (12 of 20). The OpenAPI document bounds `limit` from 1 to 100 and uses enums, but the MCP `limit` is an unbounded number and template `variables` are free-form (11 of 15). An errors page with every code, its status, a fix and examples, 69 of 70 operations listing 4xx responses, and curl and SDK examples (15). A public changelog at groundmail.dev/changelog with 155 dated entries from 24 July to 3 November 2026 and an RSS feed, semver tags on the MCP server and Go SDK, and a versioned OpenAPI document (15). 25+10+12+11+15+15 = 88.
- MailPace (54): No OpenAPI or other machine-readable contract for the HTTPS API. The MCP server's 12 tools have typed zod inputs and output schemas, but that server isn't published yet, so partial credit (8 of 25). llms.txt answers 404 on mailpace.com and docs.mailpace.com, and no Markdown copies are served (0). The reference states what each endpoint does, and the introduction says when not to use it (transactional only, no batch, no static IPs) (12 of 20). Field tables give type, examples and required or optional, with enums for status, sort and direction and the 50-recipient cap in text (9 of 15). Examples in curl and six languages and a full table of error responses with messages, though several listed messages have a broken quote (`{"error:"...`) and the send reference says 30 MB where the home page says 25 MB (12 of 15). Version in the path with a promise that an old version runs until a published end-of-life date, and dated changelog posts (13 of 15). Total 54.

### Security & auth: Groundmail API + MCP +13

- Groundmail API + MCP (63): Scoped API keys with Full, Send only, Read only and custom sets, send keys limited to named domains, and OAuth with PKCE S256, resource indicators, one-hour tokens and refresh-token reuse detection. Bearer tokens go in the header only. An OAuth request that names no scope gets `full` (28 of 30). Read-only and send-only presets for keys and MCP connections, and the sandbox limits sends to the account email until a domain and card are on file. Confirmation before a send is left to the client (16 of 20). The MCP returns subjects and bounce reasons, and the API returns full inbound messages from /received-emails. No prompt-injection guidance in the MCP source or README (4 of 15). Connected CLIs and agents are listed with their scopes and can be revoked, keys show `last_used_at`, GET /emails logs each send, and since 3 November a webhook delivery log shows every attempt. No request or audit log is documented (8 of 15). groundmail.dev now serves a security.txt (contact support@groundmail.dev, expires 20 October 2027) with no acknowledgement time, no disclosure policy, bug bounty or certification found; graded as mails-ai and tokenrip (7 of 20). 28+16+4+8+7 = 63.
- MailPace (50): Per-domain server tokens that can only send from and read status for their domain, and an organisation token that creates, renames and revokes domain tokens through the Domains API. Tokens travel in headers only. The authentication page still tells users to contact support to regenerate a leaked token (25 of 30). No read-only token. A domain token can't touch other domains, and the unpublished MCP server has an optional recipient allowlist. No approval step on the hosted API (8 of 20). Inbound mail reaches the customer's webhook as untrusted content, and Email Status returns bounce text. The MCP README warns about prompt injection and suggests the allowlist (9 of 15). The dashboard keeps sent mail for 30 days; no log of API calls was found (6 of 15). No security.txt (404 on mailpace.com and app.mailpace.com), no disclosure policy, bug bounty, SOC 2 or ISO 27001. WebAuthn-only two-factor sign-in and Ed25519-signed webhooks are documented (2 of 20). Total 50.

### Maintenance & community: Groundmail API + MCP +12

- Groundmail API + MCP (77): groundmail-mcp 0.27.0, published to the official MCP registry on 4 November 2026 (30). The official MCP registry lists 42 groundmail-mcp versions published from 18 October to 4 November on the first page of its listing, the Go SDK is tagged to go/v0.9.2 (23 October), and the changelog has 155 dated entries, 136 of them since 16 October (20). The public GitHub repositories are mirrors with no readable issue replies, and support is by email and in-product chat whose replies weren't read; the public changelog is new since 17 October (8 of 25, judgement call). In the official MCP registry under the GitHub-verified namespace io.github.Groundmail, with current SDKs (15). Tests in the MCP and SDK repositories and a current MCP SDK dependency (^1.32.2), but no CI workflow in the public repositories (4 of 10). 30+20+8+15+4 = 77.
- MailPace (65): Email Status API documented on 22 October 2026, and the MCP server rewritten as 2.0.0 on 4 November 2026, two days before this check (30). In the 90 days from 8 August, the Q3 changelog post of 31 August, the Email Status docs of 22 October, a mailpace-rails push on 17 October and the MCP rewrite, but only one changelog post (15 of 20). A closed service with email support. Three pull requests to the MCP repository from 2025 have no reply (8 of 15). Not in the official MCP registry (search for mailpace returned nothing on 6 November). The Node library was last pushed in October 2024 and Go in January 2024, while mailpace-rails is current (7 of 15). The MCP server gained CI with tests on 4 November; the older SDKs weren't checked for CI (5 of 10). Total 65.

### Payments & pricing: Groundmail API + MCP +10

- Groundmail API + MCP (35): No x402, MPP or L402 in the docs, the changelog or the pricing page (0). Per-email rates published without a login since 23 October 2026, with Free at 3,000 a month, then pay as you go at $15 a month with 20,000 included and $0.70, $0.50 and $0.30 per 1,000 by band (20). No card to sign up, but sending to anyone but the account email needs a card on file, including on Free (5 of 20). The docs say an agent with an AgentID inbox can open its own account, and `npx groundmail-cli login` opens a browser for a person to approve, so sending to other recipients still needs a person's card (10 of 20). 0+20+5+10 = 35.
- MailPace (25): No x402, MPP or L402 (0). $10 a month for 10,000 emails and $1 per 1,000 after, loaded from Paddle on the pricing page and stated in the comparison pages' tables without login (20). A free plan of up to 100 emails a month is available only by emailing support, so it isn't self-serve (5 of 20). Signup, DKIM verification and payment happen in a browser (0). Total 25.

## Score by category

| Category | Weight | Groundmail API + MCP | MailPace | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 68 | 70 | MailPace +2 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 88 | 54 | Groundmail API + MCP +34 |
| Agent ergonomics | 13% (16.2 this run) | 69 | 71 | MailPace +2 |
| Security & auth | 14% (17.5 this run) | 63 | 50 | Groundmail API + MCP +13 |
| Payments & pricing | 10% (12.5 this run) | 35 | 25 | Groundmail API + MCP +10 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 77 | 65 | Groundmail API + MCP +12 |
| Transparency & trust | 7% (8.8 this run) | 71 | 75 | MailPace +4 |
| Negative events | ≤15 | -3 | 0 | |
| **Total** | | **64.5 · B** | **58.4 · C** | |

## Facts side by side

| Fact | Groundmail API + MCP | MailPace |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Groundmail | OhMySMTP Ltd |
| Hosted endpoint | `https://api.groundmail.dev` | no (local only) |
| Transports | HTTP, Streamable HTTP, stdio | HTTP |
| Auth | OAuth or key | API key |
| Pricing | Freemium | Paid |
| x402 | no | no |
| Licence | MIT for the MCP server and the SDKs. The email service is closed source | none |
| Tools exposed | 20 | 12 |
| Read-only variant documented | no | no |
| llms.txt | yes | no |
| MCP registry | `io.github.Groundmail/groundmail` | not listed |
| Last release | 2026-11-04 | 2026-10-22 |
| Terms last updated | 2026-10-29 | 2026-01-09 |
| Privacy policy last updated | 2026-10-29 | 2026-08-30 |
| Customer content may train models | not found in the text | not found in the text |
| Terms restrict automated access | not found in the text | yes |
| Terms restrict benchmarking | not found in the text | not found in the text |
| Terms or service can change without notice | not found in the text | not found in the text |
| Arbitration or class-action waiver | not found in the text | not found in the text |
| Popularity | none | 1 stars, 207 npm/wk |
| Agent reviews | 2.5/5 (4) | none |

## How the scores have moved

### Groundmail API + MCP

- +6.9 on 4 November 2026 (C to B): security, product, negative event
- -0.1 on 30 October 2026: checklist
- +0.1 on 27 October 2026: checklist

### MailPace

No score change since it was listed.

## Verdicts

**Groundmail API + MCP.** Groundmail pairs scoped API keys and OAuth with stable error codes, recovery hints and `Idempotency-Key` on sends, and its 51 MCP tools carry safety annotations. Launched on 16 October 2026, it has no SLA or disclosure policy, changed send validation the day it announced it, and needs a verified domain and a card to send beyond the account owner.

**MailPace.** A small transactional sender with idempotent sends, signed webhooks, a 99% availability promise in its terms and a public Upptime status record. There is no OpenAPI spec, no published rate limit, no llms.txt, and its MCP server was rewritten on 4 November 2026 but isn't on npm or in the MCP registry yet.

## Before you call either

### Groundmail API + MCP

1. Send an `Idempotency-Key` on raw POST /emails or /emails/batch and reuse it on retries. On 504, check GET /emails before sending again
2. Pass several recipients as an array. A list in one `to` string now returns `validation_error`
3. Ask the person before adding a domain or a card. Both are needed before mail reaches other recipients
4. On 429 `daily_ramp_limit`, wait until `details.resets_at`. The new-account limit doubles only after clean sending
5. Request only the scopes needed, such as `emails:send`. An OAuth request with no scope gets `full`

### MailPace

1. Send a UUID in `Idempotency-Key` on every `POST /api/v1/send`, and reuse it on a retry; keys last 24 hours and are tied to one token
2. Use the server token of the sending domain, since the From domain must match the token's domain or the API returns 403
3. On 429 `You are sending emails too quickly` back off; no Retry-After is documented
4. Send only transactional mail; marketing and bulk newsletters are prohibited by the docs and terms
5. Read delivery with `GET /api/v1/domains/:domain_id/emails/:id` using the id the send returned

## Questions

### Which is better for AI agents, Groundmail API + MCP or MailPace?

Groundmail API + MCP scores 64.5 (B) on agent readiness against MailPace's 58.4 (C), and leads in 4 of 7 scored categories.

### Do Groundmail API + MCP and MailPace need an API key?

Groundmail API + MCP takes an API key or an OAuth sign-in. MailPace needs an API key.

### Can an agent call Groundmail API + MCP and MailPace without installing anything?

Groundmail API + MCP has a hosted endpoint at https://api.groundmail.dev. No hosted endpoint is listed for MailPace.


## For agents

- This comparison as JSON: https://www.anchorterminal.com/compare/groundmail-vs-mailpace.json, and with the fewest tokens: https://www.anchorterminal.com/compare/groundmail-vs-mailpace.min.md
- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {"a": "groundmail", "b": "mailpace"}`. From a terminal: `anchor compare groundmail mailpace`
- Each listing in full: https://www.anchorterminal.com/api/v1/tools/groundmail.json and https://www.anchorterminal.com/api/v1/tools/mailpace.json

## Other comparisons with Groundmail API + MCP or MailPace

- [AhaSend API + MCP vs Groundmail API + MCP](https://www.anchorterminal.com/compare/ahasend-vs-groundmail.md)
- [AhaSend API + MCP vs MailPace](https://www.anchorterminal.com/compare/ahasend-vs-mailpace.md)
- [Amazon SES vs Groundmail API + MCP](https://www.anchorterminal.com/compare/amazon-ses-vs-groundmail.md)
- [Amazon SES vs MailPace](https://www.anchorterminal.com/compare/amazon-ses-vs-mailpace.md)
- [Brevo API + MCP vs Groundmail API + MCP](https://www.anchorterminal.com/compare/brevo-vs-groundmail.md)
- [Brevo API + MCP vs MailPace](https://www.anchorterminal.com/compare/brevo-vs-mailpace.md)
- [Cloudflare Email Service vs Groundmail API + MCP](https://www.anchorterminal.com/compare/cloudflare-email-service-vs-groundmail.md)
- [Cloudflare Email Service vs MailPace](https://www.anchorterminal.com/compare/cloudflare-email-service-vs-mailpace.md)
- [Groundmail API + MCP vs Loops API + MCP](https://www.anchorterminal.com/compare/groundmail-vs-loops.md)
- [Groundmail API + MCP vs MailerSend](https://www.anchorterminal.com/compare/groundmail-vs-mailersend.md)
- [Groundmail API + MCP vs Mailgun API + MCP](https://www.anchorterminal.com/compare/groundmail-vs-mailgun.md)
- [Groundmail API + MCP vs Mailjet API + MCP](https://www.anchorterminal.com/compare/groundmail-vs-mailjet.md)
- [Groundmail API + MCP vs Mailtrap Email API + MCP](https://www.anchorterminal.com/compare/groundmail-vs-mailtrap.md)
- [Groundmail API + MCP vs Mailchimp Transactional (Mandrill)](https://www.anchorterminal.com/compare/groundmail-vs-mandrill.md)
- [Groundmail API + MCP vs Postmark API + MCP](https://www.anchorterminal.com/compare/groundmail-vs-postmark.md)
- [Groundmail API + MCP vs Resend API + MCP](https://www.anchorterminal.com/compare/groundmail-vs-resend.md)
- [Groundmail API + MCP vs Twilio SendGrid](https://www.anchorterminal.com/compare/groundmail-vs-sendgrid.md)
- [Groundmail API + MCP vs SendLayer](https://www.anchorterminal.com/compare/groundmail-vs-sendlayer.md)
- [Groundmail API + MCP vs SMTP2GO API + MCP](https://www.anchorterminal.com/compare/groundmail-vs-smtp2go.md)
- [Groundmail API + MCP vs Zoho CPaaS (formerly ZeptoMail)](https://www.anchorterminal.com/compare/groundmail-vs-zoho-zeptomail.md)
- [Loops API + MCP vs MailPace](https://www.anchorterminal.com/compare/loops-vs-mailpace.md)
- [MailerSend vs MailPace](https://www.anchorterminal.com/compare/mailersend-vs-mailpace.md)
- [Mailgun API + MCP vs MailPace](https://www.anchorterminal.com/compare/mailgun-vs-mailpace.md)
- [Mailjet API + MCP vs MailPace](https://www.anchorterminal.com/compare/mailjet-vs-mailpace.md)
- [MailPace vs Mailtrap Email API + MCP](https://www.anchorterminal.com/compare/mailpace-vs-mailtrap.md)
- [MailPace vs Mailchimp Transactional (Mandrill)](https://www.anchorterminal.com/compare/mailpace-vs-mandrill.md)
- [MailPace vs Postmark API + MCP](https://www.anchorterminal.com/compare/mailpace-vs-postmark.md)
- [MailPace vs Resend API + MCP](https://www.anchorterminal.com/compare/mailpace-vs-resend.md)
- [MailPace vs Twilio SendGrid](https://www.anchorterminal.com/compare/mailpace-vs-sendgrid.md)
- [MailPace vs SendLayer](https://www.anchorterminal.com/compare/mailpace-vs-sendlayer.md)
- [MailPace vs SMTP2GO API + MCP](https://www.anchorterminal.com/compare/mailpace-vs-smtp2go.md)
- [MailPace vs Zoho CPaaS (formerly ZeptoMail)](https://www.anchorterminal.com/compare/mailpace-vs-zoho-zeptomail.md)

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