Resend is great. It is not built for AI agents.

OpenMail vs Resend · Updated September 2026Resend is built to send. OpenMail is built for agents that send, receive, and reply in thread.

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Where they differ

FeatureOpenMailResend
Inbox per agentcreated by APIno, domain catch-all
Inbound eventbody + parsed attachmentsmetadata only; body is a second call
Threadingautomatic, thread ID in every eventmanual (you set In-Reply-To)
Retentionpersistent30 days on free and Pro
Rate limitper inboxshared per team
Data residencyEU, every planUS (sends from Ireland)
First paidEUR 9/mo, 10 inboxesUSD 20/mo, 50k emails
SOC 2in progressType II

Why sending isn’t enough

A support agent at help@yourco.com needs to hold a conversation over weeks. Here's what that looks like:

Resend

  • Body is a second API call
  • You manage Message-ID / In-Reply-To
  • 30-day retention
  • Shared rate limit across all agents

OpenMail

  • Body + attachments in the event
  • Thread ID in every event
  • Persistent retention
  • Rate limit per inbox

Pricing

Resend ProUSD 20/mo for 50,000 emails, then USD 0.90 per 1,000
OpenMail ProEUR 9/mo for 10 inboxes, then EUR 1 per inbox
Per emailResend is cheaper
Per agentOpenMail is priced per inbox

Data residency

ResendSends from Ireland; account data stored in the US
OpenMailEU on every plan, free included

Choose OpenMail if

Your agent has to receive and answer, not just send

Each agent needs its own address, reputation and rate limit

You need EU data residency without an enterprise contract

You want the inbox as the unit you build on

The same loop on OpenMail

1. Create the inbox. Live immediately.

bash
curl -X POST https://api.openmail.sh/v1/inboxes \
  -H "Authorization: Bearer $OPENMAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"mailboxName": "help", "displayName": "Acme Support", "webhookUrl": "https://yourapp.example/webhooks/openmail"}'

2. Receive. Body + attachments in the event. No second call.

json
{
  "event": "message.received",
  "inbox_id": "inbox_123abc",
  "thread_id": "thr_xyz",
  "message": {
    "from": "customer@example.com",
    "subject": "Re: my order",
    "body_text": "Still nothing three weeks later. Invoice attached.",
    "attachments": [{"filename": "invoice.pdf", "parsedText": "Invoice #2847 ..."}]
  }
}

3. Reply in thread. We set the headers. Still there in a year. This call uses an inbox-scoped key, so the agent can only send from its own inbox.

bash
curl -X POST https://api.openmail.sh/v1/inboxes/inbox_123abc/send \
  -H "Authorization: Bearer $OPENMAIL_AGENT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"to": "customer@example.com", "body": "Found it. Refund issued today.", "threadId": "thr_xyz"}'

Each inbox has its own rate limit and sending reputation. Full walkthrough: How to give your AI agent its own email address

Where Resend wins

SOC 2 Type II. Eight official SDKs. React Email. No daily cap on Pro. Custom domains on the free plan. If your agent only sends, Resend is the better tool.

Frequently asked questions.

Every AI agent deserves its own inbox.

Start building today.

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3 inboxes, 3,000 emails a month, no card required