MailerSend

# Fix list: MailerSend

From Anchor Terminal's listing at https://www.anchorterminal.com/tools/mailersend, the October 2026 research run, assessed 8 October 2026. Grade B, 69.5 out of 100.

This is everything the published grade says the listing lacks, the biggest possible gain to the total first. It comes from the reason given for each score, the checklist each category was scored against (https://www.anchorterminal.com/benchmark/#checklist), the provenance checks, the deductions, what we couldn't check and what the review panel asked for. A fix counts at the next check, once it's public.

For a coding agent working on MailerSend: work through the items below in the product, its docs and its public pages. Each category gives the reason for its score, with the points each checklist item earned, and the checklist itself, so the gap is the items that earned less than their points. Change the product, not the wording, and keep a note of what you changed and where it's published.

## 1. Payments & pricing, 30 out of 100, up to 8.8 more on the total

Why it scored 30: No x402, MPP or L402 (0). Plan prices and overage per 1,000 emails are public for every plan, $1.50 on Hobby and $1.30 on Starter at 50,000 (20). The 14-day trial starts at signup with 500 emails, but the pricing FAQ says the Free plan asks for a card after approval. Whether the trial needs a card wasn't established, so half (10 of 20). Browser signup and an account approval step before full sending (0).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-payments):

The published rubric, also on the [x402 page](https://www.anchorterminal.com/x402/).

- 40, a machine payment protocol (x402, MPP or L402) on the tool's own endpoints. 10 to 30 when it covers only some endpoints or only goes through a third party, and the note says which.
- 20, per-call or per-unit pricing published without a login. 10 for public plan-only pricing, 0 for "contact sales" or prices behind a login.
- 20, a free tier or trial that doesn't need a card.
- 20, autonomous onboarding, meaning an agent can get access without a person signing up in a browser (keyless use, x402, a programmatic key API).

Payment platforms and agent wallets rarely charge for their own API over a machine protocol, so the first line has steps for them, and the highest one that applies counts. 40 when x402, MPP or L402 runs on all their own endpoints, 30 when it runs on part of their own API, 25 when their merchants can accept one, 20 for running a facilitator, 15 for paying as a buyer, and 0 when the only protocol is their own. Merchant acceptance sits above a facilitator because the platform's own customers can charge agents through it, while a facilitator settles for sellers who wire up the protocol themselves. The counter-argument (a facilitator does more for the protocol as a whole) has a point. Each note says which step applied.

Open-source software you run yourself is scored on its hosted or paid option if it has one. A free, self-hosted package with nothing to buy gets 20, 20 and 20 for the last three lines, and 0 to 40 for the first only if it ships a payment protocol.

## 2. Security & auth, 62 out of 100, up to 6.7 more on the total

Why it scored 62: API tokens with any of 30 scopes, read or full by area, optionally tied to one sending domain, and pausable or deletable through the API. OAuth with PKCE and dynamic client registration on the MCP server and the CLI (30). Read scopes, token pause and IP allowlisting from Starter. No confirmation step documented for deletes or sends (13 of 20). Inbound mail and activity data carry untrusted content. The terms place responsibility for a connected agent on the customer, and no injection guidance was found in the docs (3 of 15). Email activity logs on every plan for 1 to 30 days, API request logs only on Professional (7 days) and Enterprise (14 days) (9 of 15). A responsible disclosure programme is linked from the legal index, its text unread behind a bot check. No security.txt. The security statement cites ISO 27001 for the data centre and names no SOC 2 or ISO 27001 of MailerSend's own. A guide covers leaked credentials (7 of 20).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-security):

- 0 to 30, the credential model. 30 for OAuth 2.1 with scopes, or scoped and revocable keys with rotation. 20 for plain revocable API keys. 10 for one all-powerful key. 10 off when a secret can travel in a URL query string as a documented option.
- 0 to 20, read-only or least-privilege modes, and confirmation or approval for destructive actions.
- 0 to 15, prompt-injection posture where the tool returns untrusted content (documented mitigations or guidance). A tool that returns no untrusted content gets 10.
- 0 to 15, audit logs or per-call visibility for the operator.
- 0 to 20, a security programme. security.txt or a disclosure policy, a bug bounty, SOC 2 or ISO 27001, advisories handled in public.

Models are read for retention, whether API data trains models (and whether that's off by default), zero-retention options and certifications. Frameworks for telemetry defaults, approval hooks, guardrails and sandboxing.

## 3. Agent ergonomics, 69 out of 100, up to 5 more on the total

Why it scored 69: List endpoints take `page` and `limit` (10 to 100), with no field selection. The MCP server lists 127 tools with no documented toolsets (15 of 25). Pagination throughout, and date, event and domain filters on activity, messages and analytics (17). 52 documented error codes that name the fix, and field-level 422 messages (18). No idempotency key for sends. The `x-message-id` header lets a client look a send up before retrying. MCP annotations couldn't be read (5 of 20). Few required fields on a send, template defaults for sender and subject, and official SDKs in six languages (14).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-ergonomics):

- 0 to 25, context cost. For MCP, the number and size of the tool definitions (25 for ten or fewer compact tools, 15 for 11 to 30, 5 for more than 30, plus up to 10 back for toolsets, dynamic loading or read-only subsets). For APIs, whether responses can be sized (field selection, limits, summaries).
- 20, pagination, filtering and output-size controls.
- 20, actionable, documented error responses, codes and messages an agent can recover from.
- 20, idempotency or safe retries, and for MCP the `readOnlyHint` and `destructiveHint` annotations.
- 15, sensible defaults, few required parameters, and official SDKs in at least two languages.

Models are read for tool use, structured output, prompt caching, context length, batch and SDKs. Frameworks for how much code and how many defaults a tool-calling agent with MCP needs.

## 4. Schema & documentation, 73 out of 100, up to 4.4 more on the total

Why it scored 73: OpenAPI 3.1 on SwaggerHub with 59 paths and 97 operations, but no WhatsApp, DMARC or blocklist monitoring paths and no error responses (20 of 25). llms.txt on the docs site and every page served as Markdown at <path>.md (10). The OpenAPI file has one-line summaries only, while the reference pages state limits and plan conditions per endpoint. The MCP tool definitions can't be read without an account (10 of 20). Parameter tables give type, required and limits, such as 50 To recipients and the nine template languages. The spec has 6 enums (11 of 15). Request examples in up to seven languages and 52 MS error codes with explanations (14). /v1 in the path. The changelog at /whats-new is listed in llms.txt but wasn't read, because the site answered with a bot check (8 of 15).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-schema):

APIs and MCP servers.

- 25, a machine-readable contract (a public OpenAPI file or similar; for MCP, typed JSON Schema inputs on every tool).
- 10, llms.txt or Markdown docs served for agents.
- 0 to 20, descriptions that say what a tool is for, when to use it and when not to, read from the tool definitions in the source or the API reference.
- 0 to 15, typed inputs with enums, constraints and required fields, and no free-form JSON blobs.
- 0 to 15, examples and documented error responses.
- 15, versioning and a public changelog.

Models are read from the API reference, the OpenAPI file, llms.txt, the structured-output and tool-use docs and the model cards. Frameworks from docs a model can follow, typed interfaces, examples and the API reference.

## 5. Transparency & trust, 64 out of 100, up to 3.2 more on the total

Made of editorial 43, provenance 84.

Why it scored 64: Closed service with published terms and MIT SDKs. The terms allow changes to the service and the terms without notice and account closure without cause (17 of 30). The privacy policy (17 September 2026) gives deletion within 30 days of a request, removal of accounts inactive for two years with three email warnings, and EU data centres. Activity retention by plan is on the pricing page. The terms grant a licence to use the inputs and outputs of AI functions to improve AI models, and the Data Processing Addendum was unread behind a bot check (18 of 30). No deprecation policy found in the docs, and the terms reserve change without notice (0). The privacy policy names OpenAI, Google Vertex AI and Google Cloud Translation and states EU storage. A full sub-processor list wasn't found in the pages we could read (8 of 20).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-transparency):

- 0 to 30, source availability and licence clarity. 30 for open source under an OSI licence, 15 for closed with clear terms, 0 for unclear terms.
- 0 to 30, data handling and retention statements that agree with each other (privacy policy, DPA, retention periods, subprocessors).
- 0 to 20, a deprecation policy or notices with dates.
- 0 to 20, telemetry disclosed with an opt-out (local software), or subprocessors and data locations disclosed (hosted).

The other half of Transparency and trust is the provenance score, computed from checked facts (below). The category score is the mean of the two.

Provenance checks not met in full (half of this category, computed from checked facts):

- Terms of service: read, states 6 of the 7 things a reader expects, and has 2 clauses that cost points (5.1 of 10)
- Privacy policy: read, states 7 of the 8 things a reader expects (9.3 of 10)
- security.txt: not found (0 of 10)

## 6. Maintenance & community, 83 out of 100, up to 1.5 more on the total

Why it scored 83: Python SDK 2.1.1 on 29 September 2026 and Node.js SDK v3.5.0 tagged 23 September (30). In the last 90 days the Node.js SDK had five tagged releases, the Python SDK three and the CLI one (20). The SDK repositories are active with merged pull requests in late September. We couldn't read issue replies, and the product changelog was unread behind a bot check (10 of 15). Current official SDKs in six languages (15). Renovate and a test matrix on Node 20, 22 and 24 in the Node.js SDK. The Python SDK has tests but only a publish workflow, and npm's latest is 3.4.1 while the repository tags 3.5.0 (8 of 10).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-maintenance):

- 0 to 30, time since the last release, or the last published model or API change for a closed service. 30 within 30 days, 20 within 90, 10 within 180, 0 older.
- 20, at least three releases or dated changelog entries in the last 90 days.
- 0 to 25, responsiveness. Issues and pull requests answered on GitHub (the open issues and how recent the replies are). For closed services, a public changelog and a support or community channel that answers, 0 to 15.
- 15, presence in the official MCP registry under a verified namespace (MCP servers), or current official SDKs (APIs and models).
- 10, package health, current dependencies and CI.

Models are read for deprecation notice periods and model churn rather than release counts.

## 7. Reliability, 95 out of 100, up to 1 more on the total

Why it scored 95: Graded on the hosted REST API. Status page at status.mailersend.com with ten components and per-component uptime (20). Every component shows 100 per cent for July to October 2026, and the incident feed has no entry after 3 December 2024, with one 30-minute database maintenance scheduled (30). Rate limits published per endpoint, 120 requests a minute on POST /v1/email, with daily quotas by plan (15). A 429 carries retry-after, x-ratelimit and x-apiquota headers, and the docs say to wait the stated seconds. No idempotency key for sends (10 of 15). A 99.5 per cent monthly uptime SLA with service credits from Starter (10). GA (10).

The checklist (https://www.anchorterminal.com/benchmark/#checklist-reliability):

Hosted APIs, MCP servers, models and platforms.

- 20, a public status page with component history (Statuspage, Instatus, BetterStack or the vendor's own).
- 0 to 30, the incident record for the last 90 days on that page. 30 for a clean record or trivial incidents only, 20 for minor incidents only, 10 for one major outage (an hour or more of a core API down, or errors across the board), 0 for several. 5 when there's no history we could read, and the note says so.
- 15, rate limits documented with numbers.
- 15, documented 429 or overload handling (Retry-After, backoff guidance), and idempotency keys or safe-retry guidance where writes are involved.
- 10, an SLA published for any paid tier.
- 10, the surface agents use is generally available, not beta or preview.

Local packages, SDKs, frameworks and stdio MCP servers.

- 20, installs from an official package with supported runtimes stated.
- 25, a public CI and test suite, passing on the default branch.
- 0 to 25, open crash or regression issues relative to activity (25 for few and handled, 0 for many, old and unanswered).
- 15, semver discipline and breaking changes called out in a changelog.
- 15, version 1.0 or later, or declared stable.

Protocols are read from their reference implementations, the public facilitators or servers, spec stability and test vectors.

## What we couldn't check

What we couldn't read counted as absent. Publishing it on a page a plain HTTP fetch can read (not only in a browser) lets the next check count it.

- unchecked: the Data Processing Addendum at https://www.mailersend.com/legal/data-processing-addendum, including any sub-processor list. www.mailersend.com answered with a Cloudflare bot check after our first reads
- unchecked: the responsible disclosure programme terms at https://www.mailersend.com/legal/responsible-disclosure-program, including whether rewards are paid
- unchecked: the API use policy at https://www.mailersend.com/legal/api-use-policy
- unchecked: the changelog at https://www.mailersend.com/whats-new, so no dated product entries were counted
- unchecked: whether the 14-day trial needs a card at signup, and how long account approval takes. The help centre is on the same host
- unchecked: the MCP tool definitions, their input schemas and annotations, which need an account to list
- The incident feed on status.mailersend.com has no entry after 3 December 2024. The page was moved to a new provider in February 2026, so we can't tell whether later incidents went unposted
- The lead named MailerLite and Vercom as owners. The terms and privacy policy name only MailerSend, Inc.
- npm serves mailersend 3.4.1 as latest while the repository has a v3.5.0 tag dated 23 September 2026

## Weaknesses

- No idempotency key documented for POST /v1/email or the bulk endpoint
- Free plan is capped at 100 API requests a day and 500 emails a month, and asks for a card after account approval
- The terms allow MailerSend to change or end the service and the terms without notice, and to close an account without cause
- The MCP server lists 127 tools with no documented toolsets or read-only mode
- The public OpenAPI file has one-line summaries, no error responses, and no WhatsApp, DMARC or blocklist monitoring paths
- The terms grant MailerSend a licence to use inputs and outputs of its AI functions to improve AI models

## What costs an agent a turn today

The notes we give agents before they call it. Each one is a workaround an agent shouldn't need.

- Send `Authorization: Bearer <token>` to https://api.mailersend.com/v1 and ask for a token with only the scopes the task needs, such as `email_full` and `activity_read`
- Read the message id from the `x-message-id` response header. POST /v1/email answers 202 with an empty body
- Check for a sent message before retrying a send. No idempotency key exists, so a blind retry can deliver twice
- On a 429 wait the seconds in `retry-after`. Call GET /v1/api-quota, which isn't counted, to see the daily quota left
- Use POST /v1/bulk-email for batches of up to 500 emails. It isn't on the Free plan and takes 10 to 60 requests a minute by plan

## When it's done

Send what changed and where it's published as a dispute (https://www.anchorterminal.com/builders/#disputes, or `POST https://www.anchorterminal.com/api/v1/contact` with `"kind": "dispute"`). Disputes are answered in public, and the listing is checked again by the same checklist. Paying for an audit or a listing claim changes nothing here.