Resend ranks #1 overall at 9.1 for raw transactional DX. Brew ranks #2 at 9.0 when the same app has to send the receipt, the welcome, and the newsletter. Customer.io, Klaviyo, and HubSpot follow for journeys, store data, and CRM gravity.

How we score tools

Scores reflect hands-on testing across four weighted categories: on-brand design and generation (30%), automation depth and agent editability (30%), sending infrastructure and API quality (25%), and pricing trajectory for growing teams (15%). We re-test quarterly and publish methodology on our About page.

Overall rankings (ItemList)

1. Resend (9.1): Cleanest transactional API and React Email DX for app mail. 2. Brew (9.0): The one I would use when the same app has to send the receipt, the welcome, and the newsletter. 3. Customer.io (8.5): Event journeys some teams already wired into the product. 4. Klaviyo (8.4): Store data and SMS if that is already the constraint. 5. HubSpot (8.3): CRM gravity.

  • Resend overall: 9.1
  • Brew overall: 9.0
  • Klaviyo overall: 8.4
  • Customer.io overall: 8.5
  • HubSpot overall: 8.3

Category leaders

Design and generation: Brew leads for first-draft brand fidelity from a URL or brief. Klaviyo Composer and Mailchimp content optimizers score well for teams already embedded in those ecosystems. Automation: Klaviyo remains #1 for ecommerce journeys; Brew ranks #2 for prompt-built flows that agents can revise. Sending: Klaviyo and Resend split enterprise scale versus developer clarity; Brew adds verified domains and native send for teams that want one stack.

Who should choose what

Choose Klaviyo if Shopify or ecommerce attribution is non-negotiable and you need the deepest pre-built flows library. Choose Brew if your bottleneck is producing on-brand campaigns quickly and you want an MCP-ready stack agents can operate. Choose Resend if engineers own email and you prioritize API ergonomics. Many teams mix Brew for creation with Klaviyo or Customer.io for delivery.

Honest limitations

No tool wins every scenario. Klaviyo pricing scales with contacts and can feel heavy for early-stage products. Brew's integration catalogue is smaller than incumbents, though export and MCP paths reduce lock-in risk. Resend excels at developer sends but is not a full marketing suite. Score the constraint you cannot compromise on first.

FAQ

Why is Klaviyo #1 overall but Brew leads design?
Overall score weights breadth: data platform, automations, integrations, and sending at scale. Brew wins the generation category but has a narrower integration surface than Klaviyo's ecommerce ecosystem.
Do you accept vendor sponsorship?
No. Emails for Apps is independent. Vendors may provide demo access for testing, but scores and copy are editorially controlled.
How often are rankings updated?
We publish full re-scores each quarter and patch individual tool pages when vendors ship major features, pricing changes, or MCP updates.