Scaling a fast-growing developer email platform with serverless workflows

Resend (YC W23) is a modern email sending platform that's raising the bar for developer experience. The team's early success building the popular React Email library helped drive initial growth, which they have continued to build on, making Resend a go-to for developers. Since their launch, Resend has been scaling quickly, which led them to explore Inngest, specifically for developer experience (DX), observability, and reliability. Game recognize game. Great DX recognizes great DX.

Verifying email domains with workflows

Resend initially adopted Inngest to power their domain verification workflow.

Verifying the domain is the key first step in sending emails on behalf of a user. It's the first thing anyone does when they set up Resend. Setting up the proper DNS records is fundamental to good deliverability. DNS record propagation can take time, so verification checks must happen periodically over a 72-hour period, about every 3-5 minutes.

Domain verification is a key onboarding flow to getting someone set up with Resend, so it must be reliable and scalable. When problems do occur, the Resend team needs to be able to quickly debug what has happened. Enter Inngest.

Overcoming challenges with observability

Resend's existing solution, a serverless queue, didn't provide any observability into their jobs. They would "understand that a job failed, but not why it failed." There was no connection between messages in their queue, their logs, and what happened. For a fast-growing product team, this was valuable time lost every day sifting through logs and debugging. They shifted this workflow to Inngest in just a few days and immediately saw the benefits:

1. Incredible developer experience enabling faster development velocity - The Resend team was able to use the Inngest Dev Server to build the domain verification workflow very quickly with an easy setup and fast feedback loops. Their previous serverless queue required running ngrok and manually sending messages, but with the Inngest Dev Server, iterating was as fast as seeing an error, fixing the code, and clicking "Re-run."
2. Faster debugging in production = better experience for customers - With the Inngest dashboard's function run history and logs, the team is now finally able to connect the dots between customer issues and their code, helping them get answers to customers and ship fixes faster than before.
3. No managing complex infrastructure - Resend has chosen a mix of serverless services to focus on the business value of the product instead of managing infrastructure, and also to scale more easily. Inngest was an ideal match for them as a managed solution that scales automatically.
4. Reliability - With such a fundamental part of onboarding, Inngest's reliability features helped Resend ship with confidence.

Expanding to core features: Email Broadcast and Batching

After moving their domain verification flow to Inngest in just a few days, the Resend team started work on a brand new high-volume batch workflow and Broadcast API.

Resend's Broadcast and Batching APIs take full advantage of Inngest's batching and throttling capabilities by enabling users to send millions of emails:

Behind Broadcasts lies a quietly powerful system that can queue, throttle, and send millions of emails, gather and display engagement metrics, and more.

Motivated by a shared love of DX and Inngest's reliability, Resend is continuing to leverage Inngest to bring new features and scale their product to 400,000+ developers and beyond.