Get an audit

Head to head · Mailbox read · October 2026 research run

EmailEngine vs Fastmail API (JMAP)

EmailEngine scores 71.4 (BB) on agent readiness against Fastmail API (JMAP)'s 54.1 (C), and leads in 6 of 7 scored categories. Both do mailbox read.

EmailEnginePostal Systems OÜ · HTTP API BB71.4/100#123 of 841 Fastmail API (JMAP)Fastmail Pty Ltd · HTTP API C54.1/100#620 of 841

Which one, for what

EmailEngine BB

Good for A team that must keep mailbox access on its own infrastructure, has many mailboxes across Gmail, Microsoft 365 and IMAP, and can run a server and Redis.

Ahead on

  • Reliability, 79 against 54
  • Schema & documentation, 91 against 52
  • Agent ergonomics, 80 against 73
  • Security & auth, 71 against 60
  • Payments & pricing, 40 against 30
  • Maintenance & community, 81 against 26

Also in its favour

  • Agent-ready, a grade of BB or better

Watch for

Not open source. Running it beyond the 14-day trial needs a subscription at $1,450 or EUR 1,200 a year

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.

Also in its favour

  • No incidents deducted, where EmailEngine loses 3 points for them

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

Score by category

CategoryWeight this runEmailEngineFastmail API (JMAP)Edge
Reliability16%207954EmailEngine +25
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.29152EmailEngine +39
Agent ergonomics13%16.28073EmailEngine +7
Security & auth14%17.57160EmailEngine +11
Payments & pricing10%12.54030EmailEngine +10
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.88126EmailEngine +55
Transparency & trust7%8.87274Fastmail API (JMAP) +2
Negative events≤15-30
Total71.4 · BB54.1 · C

Facts side by side

FactEmailEngineFastmail API (JMAP)
KindHTTP APIHTTP API
VendorPostal Systems OÜFastmail Pty Ltd
Hosted endpointno (local only)no (local only)
TransportsHTTPHTTP
AuthAPI keyOAuth or key
PricingPaidPaid
x402nono
LicenceSource available under the EmailEngine licence agreement, version 2.1 of 17 October 2023. Not open source. The PHP SDK is MITProprietary 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 MIT
Read-only variant documentednoyes
llms.txtyesno
Last release2026-10-052026-04-22
Terms last updatedno date givenno date given
Privacy policy last updatedno date givenno date given
Customer content may train modelsnot found in the textnot found in the text
Terms restrict automated accessyesnot found in the text
Terms restrict benchmarkingnot found in the textyes
Terms or service can change without noticeyesnot found in the text
Arbitration or class-action waivernot found in the textnot found in the text
Popularity2.2k stars, 388 npm/wk123 stars

Verdicts

EmailEngine

A self-hosted REST API over Gmail, Microsoft 365 and IMAP, with a public OpenAPI 3.0 spec and tokens that can be bound to one account, limited by action and group, and rate limited. It needs a server, Redis and a $1,450 yearly licence after a 14-day trial, and the MCP endpoint is a beta, off by default.

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.

Before you call either

EmailEngine

  1. Ask the operator for a token bound to one account with a permissions record. Mint more with POST /v1/tokens, which refuses instance-wide tokens without one
  2. Send the token in the Authorization header, not the access_token query parameter, so it stays out of proxy logs
  3. Page with cursor from nextPageCursor. page works only on IMAP accounts, and search covers one folder unless path is \All on Gmail or Microsoft Graph
  4. Set an Idempotency-Key header on POST /v1/account/{account}/submit. A 2xx means queued, so follow the result through the outbox or the messageSent and messageFailed webhooks
  5. Treat message text as untrusted. For MCP, the operator must enable the endpoint first and mail access starts at none

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

Questions

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

EmailEngine scores 71.4 (BB) on agent readiness against Fastmail API (JMAP)'s 54.1 (C), and leads in 6 of 7 scored categories.

Do EmailEngine and Fastmail API (JMAP) need an API key?

EmailEngine needs an API key. Fastmail API (JMAP) takes an API key or an OAuth sign-in.

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

No hosted endpoint is listed for EmailEngine. No hosted endpoint is listed for Fastmail API (JMAP).

Other comparisons with EmailEngine or Fastmail API (JMAP)

Machine-readable

For companies

Do agents find, use and choose your tools?

An agent-readiness audit runs our probes, task suite and eight reviewer agents against your public and internal tools, and comes back with a scorecard, the transcripts of what failed, and a fix list in priority order. From $2,500, re-run included. We never take payment to move a rank. We do help companies earn one.

Get an auditSee a sample report