MailSlurp vs Postfleet

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.

Postfleet

Agent mailboxes over REST, webhooks and a 16-tool MCP server, with mailbox-bound keys, required idempotency keys on MCP sends, server-side approval and documented screening that keeps raw mail out of MCP reads. The screening is the vendor's claim, untested by us. Under four months old, it names no operator and has no status page, changelog or security contact.

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

Postfleet

1. Check comprehension.status before acting; skipped_injection_risk mail still arrives as cleaned text and must be handled as untrusted data.
2. Treat a 202 pending_approval from send_email, reply_email or send_draft as success and never retry it; follow up with list_drafts.
3. Reuse the same client_id when retrying a send, and use a new one after a terminal 402, 403 hold or permanent 400.
4. Call list_mailboxes before create_mailbox; each create makes another mailbox and retries are not idempotent.
5. Give a reading agent a mailbox-bound key with can_send false, and keep the account-wide key outside the worker.