# Resend vs Customer.io

An email API for developers against a multichannel platform built on your data.

Canonical: https://brew.new/compare/resend-vs-customerio

## The short version

Choose Resend if your developers send email from code and you want listed prices, React Email and an MCP server your agent can send through.

Choose Customer.io if your journeys need SMS, push or WhatsApp next to email, or your data is built on accounts and objects from a warehouse.

Choose Brew if you want a fully agentic setup, where your AI agent drives lifecycle marketing to engage and retain customers. Brew designs every email from your brand and runs your automations agentically.

## Resend vs Customer.io, side by side

### Key differences

| | Resend | Customer.io | Brew |
| --- | --- | --- | --- |
| Billed on | Emails for transactional; contacts for marketing | Profiles: people and objects | Emails sent and AI credits |
| Fully agentic | Partly: Your agent sends and builds flows; it writes the HTML itself | Partly: Assists by default; your team does the work in the app | Your agent designs, sends and runs automations end to end |
| Your agent takes it live | MCP server sends emails and broadcasts | Partly: Drafts by default; sending needs an admin to allow live access | Sends, schedules and runs automations; OAuth and org-scoped MCP require confirmation, brand API keys do not |
| Designs finished, on-brand emails | Partly: Editor AI drafts emails; 100 AI credits a month on Pro | Suggests copy and brand colors; you design each email | Layout, copy and images from your brand, ready to send |
| Unlimited contacts | Marketing priced by contacts, 5,000 for $40/month | Priced per profile stored | On every plan; you pay for sends |

### Creating emails

| | Resend | Customer.io | Brew |
| --- | --- | --- | --- |
| Remembers your brand for every email | Partly: Global email theme; AI can reuse past emails as reference | Partly: Brand colors and type from your domain | Brand files your agent reads every time |
| Visual editor | No-code editor for broadcasts and templates | Design Studio, drag-and-drop and code editors | Infinite canvas, edit by hand or in chat |
| Bring the emails you already have | HTML through the API or code view; React Email | HTML in the code editor | HTML, MJML, React Email, Figma or a forwarded email |

### Automation

| | Resend | Customer.io | Brew |
| --- | --- | --- | --- |
| Event-triggered automations | Custom events from your app and contact changes | Events or segment membership | Stripe, Shopify, Clerk, Supabase or your own events |
| Builds an automation from a description | AI generates automations in the dashboard | AI Agent configures the automation | Emails, waits and branches together |

### Channels

| | Resend | Customer.io | Brew |
| --- | --- | --- | --- |
| Email from your own domain | Transactional API and marketing Broadcasts | Marketing and transactional | Marketing and transactional |

### Agents and developers

| | Resend | Customer.io | Brew |
| --- | --- | --- | --- |
| MCP server for AI agents | Emails, broadcasts, contacts and automations | Full API access; read-only by default | Every Brew tool, in Claude, ChatGPT, Cursor and Grok |
| API and CLI | REST API, SMTP relay, official SDKs and a CLI | API, CLI and webhooks | REST API, typed SDK and CLI |

### Pricing

| | Resend | Customer.io | Brew |
| --- | --- | --- | --- |
| Getting started | Free plan: 3,000 emails a month, 100 a day, 1,000 contacts | A year free for startups that raised under $10M | Free trial with 500 AI credits and 1,000 sends |
| First paid plan | Pro, $20/month: 50,000 transactional emails | Essentials, price on request: 1 million emails a month | Growth, $99/month: 10,000 AI credits and 50,000 sends |
| Next plan | Pro marketing, from $40/month for 5,000 contacts | Standard and Premium, price on request: adds SMS, WhatsApp and LINE | Pro, $249/month: 25,000 AI credits and 200,000 sends |
| Top plan | Custom volume, SSO and a dedicated CSM | Custom volume, dedicated infrastructure and HIPAA | Custom volume and a dedicated account manager |

### Security and compliance

| | Resend | Customer.io | Brew |
| --- | --- | --- | --- |
| SOC 2 Type II | Certified | Certified, with ISO 27001 | Not yet; security review and DPA on request |

## Where Resend is stronger

- Prices listed publicly: Pro is $20/month for 50,000 transactional emails, and marketing starts at $40/month for 5,000 contacts. Customer.io’s are on request.
- Resend’s MCP server sends emails and broadcasts. Customer.io’s MCP is read-only by default, and sending needs an admin to allow live access.
- A free plan with 3,000 emails a month for anyone. Customer.io’s free year is for startups that raised under $10M.

## Where Customer.io is stronger

- SMS, push and in-app next to email, with WhatsApp and LINE on Premium.
- Accounts and other objects with relationships, and data pipelines built in.
- SOC 2 Type II and ISO 27001 certified. Resend holds SOC 2 Type II and no ISO 27001 certificate.

## Where both fall short

Neither designs a finished, on-brand email. Resend’s MCP server sends HTML your agent writes itself; Customer.io suggests copy and brand colors while your team designs in Design Studio.

Both price on the size of your list: Resend’s marketing plans on contacts, Customer.io on stored profiles.

## Pricing at the same list size

Resend lists its prices. Pro is $20/month for 50,000 transactional emails, marketing starts at $40/month for 5,000 contacts on a separate plan, and the free plan covers 3,000 emails a month and 1,000 contacts.

Customer.io prices every plan on request and bills on profiles, people and objects. Essentials includes 1 million emails a month, Standard adds SMS and Premium adds WhatsApp and LINE. Startups that raised under $10M can get a year free.

Brew bills on emails sent and AI credits, with unlimited contacts: Growth is $99/month for 50,000 sends and 10,000 AI credits.

## When to consider Brew

Brew is fully agentic: your agent designs on-brand emails, sends them and runs your automations. Unlimited contacts on every plan. OAuth and organization-scoped MCP connections ask you to confirm live sends and launches. Brand-scoped API keys can execute permitted calls without the MCP confirmation step.

- [Brew vs Resend](https://brew.new/compare/brew-vs-resend.md)
- [Brew vs Customer.io](https://brew.new/compare/brew-vs-customerio.md)

## Your agent runs your lifecycle email.

Connect Claude, ChatGPT, Cursor or Grok over MCP and your agent gets every Brew tool. It designs emails from your brand, builds audiences, runs automations and schedules sends. OAuth and organization-scoped MCP connections ask you to confirm live sends and launches. Brand-scoped API keys can execute permitted calls without the MCP confirmation step.

## Every email designed from your brand.

Paste your website and Brew writes your brand into two files your agent reads for every email. Describe the email and it appears on the canvas, ready to edit by hand or in chat.

## Questions

### Is Resend or Customer.io better for transactional email?

Resend, for most teams that send from code: a REST API, SMTP relay, official SDKs and React Email, with Pro at $20/month for 50,000 emails. Customer.io also sends transactional email, and fits when the same journeys need SMS, push or in-app, or run on objects synced through its data pipelines.

### Which is cheaper, Resend or Customer.io?

Only a Customer.io quote can tell you. Resend lists its prices: $20/month for 50,000 transactional emails, and marketing from $40/month for 5,000 contacts. Customer.io prices every plan on request and bills on stored profiles, people and objects; startups that raised under $10M can get a year free.

### Which is better for AI agents, Resend or Customer.io?

Resend lets agents act on their own: its MCP server sends emails and broadcasts and builds automations, though the agent writes each email’s HTML itself. Customer.io’s AI Agent configures automations and suggests copy and brand colors, and its MCP server has full API access but is read-only by default, writes drafts, and needs an admin to allow sending. Neither designs a finished, on-brand email; Brew does.

### Is there an alternative to Resend and Customer.io?

Brew, if you want an agent to design and run your email. Your agent designs finished, on-brand emails from your brand, sends them and runs your automations, with marketing and transactional email on one plan and unlimited contacts. Developers keep a REST API, typed SDK and CLI. Brew is email only, so SMS and push stay in another tool.

### Can I use Brew with Resend or Customer.io?

Yes. Export any finished Brew email as HTML and send it through Resend’s API, or paste it into Customer.io’s code editor; a direct Customer.io integration is coming soon. You can also keep Resend for the transactional email your code sends and run marketing and lifecycle email in Brew.

## Sources and review

Brew publishes this comparison. Brew Team reviewed the linked first-party documentation. This is a documentation review, not a claim of hands-on testing of every product. Pricing and plan limits can change.

Work at Resend or Customer.io? If anything on this page is wrong or out of date, email support@brew.new and we will correct it. We want to compete on the product, not on a mistaken comparison.

- [Resend pricing](https://resend.com/pricing)
- [Resend MCP server](https://resend.com/docs/mcp-server)
- [Resend connector for Claude](https://resend.com/changelog/claude-connector)
- [Resend Broadcast editor](https://resend.com/docs/dashboard/broadcasts/editor)
- [Resend usage limits](https://resend.com/docs/api-reference/rate-limit)
- [Resend SOC 2](https://resend.com/security/soc-2)
- [Resend Enterprise](https://resend.com/enterprise)
- [Customer.io pricing](https://customer.io/pricing)
- [Customer.io messaging overview](https://docs.customer.io/messaging/journeys-overview/)
- [Customer.io email editors](https://docs.customer.io/messaging/channels/email/editors/choose/)
- [Customer.io MCP server](https://docs.customer.io/ai/mcp/get-started/)
- [Customer.io AI features](https://customer.io/ai)
- [Customer.io security and compliance](https://customer.io/security)
- [Brew vs Resend](https://brew.new/compare/brew-vs-resend)
- [Brew vs Customer.io](https://brew.new/compare/brew-vs-customerio)
- [Brew pricing](https://brew.new/pricing)
- [All Brew comparisons](https://brew.new/compare)

Updated October 2026