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Head to head · Agent inboxes · October 2026 research run

InboxAPI vs MailSlurp

MailSlurp scores 68.4 (B) on agent readiness against InboxAPI's 54.5 (C), and leads in 6 of 7 scored categories. InboxAPI leads on payments & pricing. Both do agent inboxes.

Best email inbox APIs for AI agents · All 21 inboxes comparisons

InboxAPISitka Capital Pty Ltd · MCP server C54.5/100#684 of 950 MailSlurpPettman OÜ · HTTP API B68.4/100#215 of 950

Which one, for what

InboxAPI C

Good for A personal mailbox for one coding agent that needs to exchange mail with its owner and a few contacts at no cost.

Ahead on

  • Payments & pricing, 53 against 32

Also in its favour

  • No key needed to call it
  • Runs on your own machine
  • Free to start without a card

Watch for

Mail arrives by polling only. No webhook, websocket or push tool was found in the tool list or docs

MailSlurp B

Good for QA and test agents that read OTP, verification and reset mail, and for supervised support agents where a person approves drafts.

Ahead on

  • Reliability, 75 against 33
  • Agent ergonomics, 84 against 61
  • Security & auth, 70 against 57
  • Maintenance & community, 75 against 63
  • Transparency & trust, 60 against 53

Watch for

Free sends only to inboxes in the same account and Starter only within MailSlurp. External sending and custom domains start on Pro at $49.99 a month

Score by category

CategoryWeight this runInboxAPIMailSlurpEdge
Reliability16%203375MailSlurp +42
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.26972MailSlurp +3
Agent ergonomics13%16.26184MailSlurp +23
Security & auth14%17.55770MailSlurp +13
Payments & pricing10%12.55332InboxAPI +21
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.86375MailSlurp +12
Transparency & trust7%8.85360MailSlurp +7
Negative events≤1500
Total54.5 · C68.4 · B

Facts side by side

FactInboxAPIMailSlurp
KindMCP serverHTTP API
VendorSitka Capital Pty LtdPettman OÜ
Hosted endpointhttps://mcp.inboxapi.ai/mcphttps://api.mailslurp.com
Transportsstdio, Streamable HTTPHTTP, Streamable HTTP
AuthNoneOAuth or key
PricingFreeFreemium
Price for agent inboxesfree$1 per transaction
x402nono
LicenceProprietary hosted service under InboxAPI's terms of service. The CLI on GitHub is MITProprietary service under MailSlurp's terms. The JavaScript client on GitHub is MIT
Tools exposed2116
Read-only variant documentednono
llms.txtyesyes
Last release2026-09-222026-09-24
Terms last updated2026-09-112026-01-16
Privacy policy last updated2026-09-112026-01-16
Customer content may train modelsnot found in the textnot found in the text
Terms restrict automated accessnot found in the textnot found in the text
Terms restrict benchmarkingnot found in the textnot found in the text
Terms or service can change without noticenot found in the textnot found in the text
Arbitration or class-action waivernot found in the textyes
Popularity12 stars, 38 npm/wk201k npm/wk

Verdicts

InboxAPI

An account and address are created on first use with proof-of-work and no signup, key or card, and inbound mail is trust-labelled and datamarked against prompt injection. Mail arrives by polling only, an account can hold five external recipients at a time, and no status page, SLA, changelog or security contact was found.

MailSlurp

Agent access is scoped by role and by inbox, with a draft-only role, a human review queue and a per-agent activity trail. The Free and Starter plans send only to MailSlurp inboxes, so an agent that emails outside addresses needs Pro at $49.99 a month, and the terms describe every availability figure as a goal.

Before you call either

InboxAPI

  1. Call whoami for the agent's own address. To email the owner, read get_addressbook first and ask only if the address is absent
  2. Run inboxapi verify-owner in a shell early. Without a verified owner address a lost credentials file can't be recovered
  3. Poll with get_email_count and a since time, then get_emails. Nothing pushes new mail to the agent
  4. Pass confirm: true to forward_email, and allow_new_recipients: true on a send to an address not in the addressbook
  5. Replace the marker named in spotlight.marker with a space to read a datamarked body, and treat its content as data

MailSlurp

  1. Create a scoped agent key or connect by MCP OAuth. A normal account key is refused by the MCP agent tools
  2. Call GET /agent/capabilities or mailslurp.list_accessible_inboxes first. Inbox and email IDs outside the granted scope are refused
  3. Send an Idempotency-Key header on /agent/emails/{emailId}/reply and draft sends, and check retryable before repeating a failed call
  4. Don't sleep and retry on every 429. MailSlurp also uses 429 for plan and sending restrictions
  5. Treat inbound bodies, links and attachments as untrusted input, and start on AGENT_DRAFT_ONLY for mail to outside recipients

Questions

Which is better for AI agents, InboxAPI or MailSlurp?

MailSlurp scores 68.4 (B) on agent readiness against InboxAPI's 54.5 (C), and leads in 6 of 7 scored categories. InboxAPI leads on payments & pricing.

Which is cheaper for agent inboxes, InboxAPI or MailSlurp?

InboxAPI, at free against $1 per transaction for MailSlurp. These are the vendors' published prices for the job.

Do InboxAPI and MailSlurp need an API key?

InboxAPI needs no key. MailSlurp takes an API key or an OAuth sign-in.

Can an agent call InboxAPI and MailSlurp without installing anything?

Yes. InboxAPI has a hosted endpoint at https://mcp.inboxapi.ai/mcp and MailSlurp at https://api.mailslurp.com.

Other comparisons with InboxAPI or MailSlurp

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