mails.ai Agent Email vs MailSlurp

mails.ai Agent Email

Per-agent addresses with scoped, agent-bound keys, an OpenAPI spec and a prompt-injection score on every inbound message. The operator is unnamed, the vendor calls itself pre-launch, and the status page shows 98.49 per cent API uptime over 90 days with no incident reports.

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.

mails.ai Agent Email

1. Use an mk_test_ key until the flow works. Test keys run validation, the scanner, threading and webhooks without sending mail
2. Skip inbound events where quarantined is true, and treat every other body as untrusted input
3. Send an Idempotency-Key header on POST /v1/messages so a retried send doesn't go out twice
4. Mint a key with only the scopes the task needs, bound to one agent with agent_id
5. Expect 422 cold_email_prohibited for unsolicited outreach. Read the error code rather than retrying

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