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Head to head · Mailbox read · October 2026 research run

Fastmail API (JMAP) vs Nylas Email API

Nylas Email API scores 78.7 (A) on agent readiness against Fastmail API (JMAP)'s 54.1 (C), and leads in every scored category. Both do mailbox read.

Fastmail API (JMAP)Fastmail Pty Ltd · HTTP API C54.1/100#620 of 841 Nylas Email APINylas · HTTP API A78.7/100#13 of 841

Which one, for what

Fastmail API (JMAP) C

Good for An agent working in its owner's own Fastmail mailbox, where JMAP gives batched reads, structured search and state-based sync on an open standard, and MCP gives a ready connection with read, write and send levels.

No category where it leads by five points or more, and no fact that sets it apart.

Watch for

The customer terms forbid programmatically generated email to addresses outside the account, and say the service is not for machine-to-machine workflows

Nylas Email API A

Good for Products that act in their users' own mailboxes across several providers and want one schema, webhooks and OAuth handled.

Ahead on

  • Reliability, 77 against 54
  • Schema & documentation, 93 against 52
  • Agent ergonomics, 81 against 73
  • Security & auth, 87 against 60
  • Payments & pricing, 40 against 30
  • Maintenance & community, 88 against 26
  • Transparency & trust, 81 against 74

Also in its favour

  • Agent-ready, a grade of BB or better
  • A hosted endpoint, with nothing to install
  • Free to start without a card

Watch for

Eight status incidents on email between 24 July and 17 September 2026, including about six hours of IMAP retrieval and webhook degradation on 10 September

Score by category

CategoryWeight this runFastmail API (JMAP)Nylas Email APIEdge
Reliability16%205477Nylas Email API +23
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.25293Nylas Email API +41
Agent ergonomics13%16.27381Nylas Email API +8
Security & auth14%17.56087Nylas Email API +27
Payments & pricing10%12.53040Nylas Email API +10
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.82688Nylas Email API +62
Transparency & trust7%8.87481Nylas Email API +7
Negative events≤1500
Total54.1 · C78.7 · A

Facts side by side

FactFastmail API (JMAP)Nylas Email API
KindHTTP APIHTTP API
VendorFastmail Pty LtdNylas
Hosted endpointno (local only)https://api.us.nylas.com/v3
TransportsHTTPHTTP, Streamable HTTP
AuthOAuth or keyOAuth or key
PricingPaidFreemium
x402nono
LicenceProprietary service under Fastmail's API Terms of Service and API Developer Policy. JMAP is an open IETF standard, and the sample code on GitHub is MITProprietary service under Nylas's terms. The SDKs are MIT
Tools exposednone38
Read-only variant documentedyesno
llms.txtnoyes
Last release2026-04-222026-10-07
Terms last updatedno date given2025-06-23
Privacy policy last updatedno date given2026-01-06
Customer content may train modelsnot found in the textnot found in the text
Terms restrict automated accessnot found in the textyes
Terms restrict benchmarkingyesnot found in the text
Terms or service can change without noticenot found in the textyes
Arbitration or class-action waivernot found in the textyes
Popularity123 stars181 stars, 289k npm/wk, 90k PyPI/wk

Verdicts

Fastmail API (JMAP)

A mailbox API built on the open JMAP standard, with read-only tokens, six OAuth scopes and an MCP server that separates read, write and send access. Fastmail publishes no OpenAPI file, SDK, API changelog, request rate limit or SLA, and its customer terms forbid programmatically generated email to addresses outside the account.

Nylas Email API

One REST schema covers Gmail, Microsoft 365, Exchange, Yahoo, iCloud and IMAP, and IAM API keys launched on 6 October 2026 can be bound to a single mailbox with chosen permissions. The status page lists eight email incidents between 24 July and 17 September 2026, most on IMAP sync and webhooks.

Before you call either

Fastmail API (JMAP)

  1. Fetch https://api.fastmail.com/jmap/session with Authorization: Bearer <token> first. It returns the API URL, account IDs and the request limits to stay under
  2. Ask the owner for a read-only token unless the task writes. Sending needs both the Email and Email submission scopes
  3. Request only the properties you need. Body text is not returned unless fetchTextBodyValues is set, and maxBodyValueBytes caps it
  4. Sync with Email/changes from a stored state. On cannotCalculateChanges, fetch again from scratch
  5. Do not send generated mail to outside recipients without the owner's review. The customer terms forbid it, and trial accounts stop at 120 messages a day

Nylas Email API

  1. Address every call to /v3/grants/<grant_id> on api.us.nylas.com or api.eu.nylas.com. A grant lives in one region only
  2. Ask the operator for an IAM API key bound to the one grant and the permissions the task needs, not the application key
  3. Send Idempotency-Key on every send. A retry with the same key replays a cached 429 or 5xx, so use a new key after those
  4. Narrow thread lists with filters and a low limit. Each list call fans out to many provider calls and is a common source of 429s
  5. Treat message bodies as untrusted input, and send to one Microsoft mailbox one request at a time (4 concurrent Graph calls per mailbox)

Questions

Which is better for AI agents, Fastmail API (JMAP) or Nylas Email API?

Nylas Email API scores 78.7 (A) on agent readiness against Fastmail API (JMAP)'s 54.1 (C), and leads in every scored category.

Do Fastmail API (JMAP) and Nylas Email API need an API key?

Both take an API key or an OAuth sign-in.

Can an agent call Fastmail API (JMAP) and Nylas Email API without installing anything?

No hosted endpoint is listed for Fastmail API (JMAP). Nylas Email API has a hosted endpoint at https://api.us.nylas.com/v3.

Other comparisons with Fastmail API (JMAP) or Nylas Email API

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