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An AgentMail alternative with custom domains on the free plan

AgentMail gives AI agents inboxes, and it does that well. People look elsewhere for a custom domain on the free plan, mail without a footer, or sending and support inboxes in the same product.

Updated 4 min read

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This page is for people already on AgentMail, or close to choosing it, who want to know what else is out there. For a feature by feature table, read Mailbay vs AgentMail; here we cover why people switch, when they should not, and the actual steps to move. More pages like this one are on the alternatives index.

Why people look for an alternative

  • No custom domain on the free plan. AgentMail's plan limits list the free plan with 3,000 emails, 3 inboxes and “None” for custom domains [2]. An agent that writes to customers from agents.yourcompany.com needs the $20 Developer plan [1].
  • A footer on free mail. The pricing table ticks “No ‘Sent via AgentMail’ footer” only for paid plans [1].
  • Daily and burst caps. The free plan sends 100 emails a day; Developer sends 1,000 a day and 100 every 5 minutes [1]. AgentMail does not publish its request rate limits as numbers, and says polling an inbox in a tight loop is the most common way to hit them [2].
  • Agent mail only. AgentMail is built for agent inboxes. Teams that also send app email or run a support address for people end up with a second provider.

When to stay with AgentMail

Switching costs time. Stay where you are if these matter more to you than the reasons above:

  • A larger MCP server with OAuth. AgentMail's MCP server signs in with OAuth or an API key and has tools for drafts and allow and block lists [3]. Mailbay's uses an API key; OAuth sign in is planned, not built.
  • SDKs. AgentMail has open source Python and Node SDKs [4]. Mailbay's Node SDK is coming to npm, and there is no Python SDK.
  • Maturity and scale. AgentMail has paid plans you can buy today, up to 150 inboxes on Startup, plus an enterprise tier with dedicated IPs and an EU region [1]. Mailbay is in private beta, billing is not live, and our sending IPs are still warming up.

How Mailbay differs

Mailbay is in private beta and free while it runs. The Mailbay prices below are the planned plans from our pricing page; billing is not live yet.

  • Custom domains on the free plan. 3 domains with 3 inboxes and 3,000 emails a month. The planned Pro plan is $9 a month for 25 inboxes, 50,000 emails and 10 domains.
  • Addresses without creating inboxes. Every workspace gets a private mailbay.email domain whose Catch-all receives mail for any address under it. An agent can sign up as run-42@… and wait for that exact address, and the Catch-all does not count toward the inbox limit. See how the address space works.
  • A wait that returns the code. wait_for_email in MCP, and GET /v1/inboxes/{id}/wait in the API, block until a matching message arrives and return its one time codes and links. No polling loop to rate limit.
  • Sending and support in the same project. A Resend shaped POST /v1/emails for app mail, SMTP submission, and a shared support inbox with IMAP for people, next to the agent inboxes.
The Mailbay inbox filtered to the Catch-all, showing three sign up code emails sent to addresses that were never created
The Catch-all in the Mailbay dashboard: three sign up emails sent to addresses that were never created, all received in one place. Test data on example.org.

Other options

Mailbay is not the only route. OpenMail also builds inboxes for agents, with allow and block lists we do not have. Resend now offers inboxes on your own domain, in private beta [8]; our Resend alternative page covers what that means. If you only need to read your own mailbox from an agent, a Gmail MCP server may be enough.

Moving from AgentMail

  1. Keep what you need. List inboxes with GET https://api.agentmail.to/v0/inboxes [4] and each inbox's messages with GET /v0/inboxes/{inbox_id}/messages, paging with page_token [5]. The raw message endpoint returns a signed download_url that expires, so download the files rather than storing the links [6].
  2. Add your domain to Mailbay and publish the records it shows. If AgentMail receives mail on that domain today, use a new subdomain first so nothing is lost while you test (see domains that already receive mail). Before an agent writes to customers from it, send a test email to the deliverability test to confirm the domain passes SPF, DKIM and DMARC.
  3. Recreate the inboxes your agents use, or let them use addresses on the Catch-all.
  4. Point the agent at Mailbay's MCP server. In Claude Code it is one command, shown with a full run in the Claude Code integration; Cursor takes a five line config. Map tool names in prompts: Mailbay's are create_inbox, send_email, reply, list_messages and wait_for_email.
  5. Move webhooks. AgentMail signs webhooks through Svix, with svix-id, svix-timestamp and svix-signature headers [7]. Mailbay uses the Standard Webhooks scheme with webhook-id, webhook-timestamp and webhook-signature; the verification code is a small change.
  6. Switch MX for the real domain once the agents work on the test subdomain.

The Mailbay side of an agent sign up, which we ran against a local Mailbay stack:

Create an inbox, then wait for the code
curl -X POST https://api.mailbay.dev/v1/inboxes \
  -H "Authorization: Bearer $MAILBAY_API_KEY" \
  -H "Content-Type: application/json" -d '{"local_part": "signup-bot"}'

curl "https://api.mailbay.dev/v1/inboxes/$INBOX_ID/wait?subject=code&timeout=60" \
  -H "Authorization: Bearer $MAILBAY_API_KEY"
# {"message": {...}, "codes": ["590214"], "links": ["https://..."]}
Mailbay has no import endpoint for old mail. Keep your AgentMail export, or leave the old inboxes running until you no longer need their history.

Sources, read on October 14, 2026

  1. [1]AgentMail pricing: www.agentmail.to/pricing
  2. [2]AgentMail, rate limits and plan limits: docs.agentmail.to/knowledge-base/rate-limits
  3. [3]AgentMail, MCP: docs.agentmail.to/integrations/mcp
  4. [4]AgentMail, API reference: docs.agentmail.to/api-reference
  5. [5]AgentMail, list messages: docs.agentmail.to/api-reference/inboxes/messages/list
  6. [6]AgentMail, get raw message: docs.agentmail.to/api-reference/inboxes/messages/get-raw
  7. [7]AgentMail, webhook verification: docs.agentmail.to/webhook-verification
  8. [8]Resend, create inbox (private beta): resend.com/docs/api-reference/inboxes/create-inbox

Prices change. If something here is out of date or wrong, email hello@mailbay.dev and we will fix it.

Setting up agents from scratch? Mailbay for AI agent builders has the one paste for each tool.

Questions

Is there a free AgentMail alternative with custom domains?

Mailbay's free plan includes 3 custom domains, 3 inboxes and 3,000 emails a month, and Mailbay is free for everyone while the private beta runs. AgentMail's own plan limits list no custom domains on its free plan.

Can I keep my agent's prompts and tool calls when I switch?

Mostly. Both have MCP tools to create an inbox, send, reply and list messages, but the names and arguments differ. Mailbay's wait_for_email returns the codes and links already extracted, so the agent needs one call instead of polling.

Can I import my AgentMail messages into Mailbay?

Not yet. Mailbay has no import endpoint. Export the raw messages you need from AgentMail's API before you move, and point new mail at Mailbay inboxes.

Does Mailbay have a Python or Node SDK like AgentMail?

A Node and edge SDK is coming to npm; there is no Python SDK. Mailbay also has a REST API, a CLI and an MCP server, and the REST API works with plain requests or fetch, as the Python integration page shows.

Read next

Join the beta

Mailbay is in private beta and free while the beta runs: inboxes, sending, your own domains and MCP. We invite people in small batches and read every note on the form.

Join the betaRead the docs