Designing for Agents — Zeno Rocha, Resend

The host introduces Zeno Rocha, recalling his early VS Code theme and indie-hacker work. Rocha says people still recognize him more readily for creating Dracula than for Resend, although that is changing. He describes himself as a former front-end and HTML5 developer with no infrastructure or email background. He chose email after using Mailgun, Mailchimp, SendGrid, and similar services and feeling that they served marketers and product managers better than developers. His initial ambition was to build the “Stripe of email”: a developer-first platform that cared about the sending experience rather than simply competing on the existing market’s terms.

Rocha says Resend now powers email for 4.3 million users and has more than 120,000 paid customers, ranging from indie hackers to Warner Bros. and HBO. He explains that the company’s customer-wall logos began as a search for social proof: on a random Saturday, he inspected domains in Resend’s database and discovered WBD.com, realizing Warner Bros. Discovery was already using the service. The host, who invested after receiving a concise memo arguing that Mailchimp and Postmark had exited and SendGrid was inside Twilio, says he encountered Resend through an AI coding tool that recommended it even after Cloudflare email was configured.

Rocha explains that high-volume email cannot be scaled like an ordinary application by adding compute. Gmail, Outlook, and Yahoo apply different rules, throttle large batches, and evaluate early engagement before deciding how to treat the remaining messages. Resend therefore queues emails, spaces batches across providers, prioritizes more engaged recipients, buys and manages different IP blocks, examines the history of IP pools, and balances traffic over time. The process is heavily automated, but the company learned its limits gradually. New customers must establish a consistent pattern of increasing sends rather than sending to a huge audience once and disappearing, because abrupt bursts resemble abuse.

Resend initially removed the industry’s usual human-verification gate, where senders disclose volume and purpose and wait up to two days for approval—often to be denied. Rocha now understands why competitors do this: fraud, phishing, and abuse are widespread. He describes a Japanese gang that brought roughly 20,000 recently registered domains across different TLDs and registrars. Resend checks domain age, website status, lookalike domains, bounce and complaint rates, sending patterns, and provider feedback. Its philosophy remains “born free”: trust users by default, then block them quickly when behavior is abnormal, distinguishing accidental misuse from deliberate abuse and talking to customers when possible.

Rocha says Resend added product insights, link and spelling checks, and automatic plain-text generation because users often do not want to study email best practices. Converting HTML into a plain-text version improved deliverability because filters can inspect the content without parsing complex markup. The host points out that plain text removes open-rate tracking. Rocha agrees that open tracking—usually a transparent one-by-one GIF—is less reliable after Apple’s privacy changes and because filters may load messages before delivery. Click tracking is also complicated: links and image URLs should match the sending domain, while shared tracking domains create “noisy neighbor” risk if one abusive customer gets the domain blacklisted.

Rocha describes support as having gone through several versions. At first he simply wanted anyone answering tickets; later he hired people with domain expertise and imposed a rule that every response had to link to a new knowledge-base article or documentation. That made individual answers slower—possibly an hour and a half—but converted recurring support work into reusable documentation. After passing one million users, staffing alone stopped scaling. DNS became the dominant source of confusion, especially for newer users from tools such as Lovable, Bolt, and v0 who do not know what CNAME, A, or TXT records mean. Resend adopted Domain Connect, an OAuth-like protocol that lets users authorize DNS changes without manually entering records.

Rocha says customer research revealed that creating email templates was itself painful: designers supplied Figma files, developers converted them into HTML tables with inline CSS, and Outlook required special syntax. Resend created React Email as an open-source top-of-funnel project, with documentation showing how to use it alongside SendGrid, Mailchimp, and Mailgun. React is used primarily as a templating and compilation system, not for state or JavaScript interactivity, since email cannot ship JavaScript. TypeScript validation and reusable components make templates more reliable. The strategy was to give away the template solution first, then offer Resend as the platform for sending those templates.

A conversation with Mailgun’s founder taught Rocha that 90% of Mailgun’s R&D had gone toward fighting abusers, convincing him that Resend had to automate safety rather than build a huge abuse-focused organization. He considers SendGrid the historic behemoth, so surpassing its npm SDK download count earlier in the year—and later reaching more than twice its downloads—felt extraordinary, though SendGrid still sends more email through large enterprise customers. Rocha argues Twilio has neglected the developer-oriented identity behind “Ask Your Developer.” He also says AWS SES is inexpensive and useful, but users must assemble SNS for events, CloudWatch for logs, SQS for queues, and additional infrastructure around it.

After initially building on top of Amazon SES, Resend is now developing its own sending infrastructure “down to the metal.” Rocha says the project has taken months because it involves buying and managing IP blocks, controlling throughput into each mailbox provider, and maintaining provider relationships. Owning more of the stack should improve throughput, deliverability, and observability: instead of merely trusting that an email was delivered, Resend wants to show more precisely what happened, including whether messages reached the inbox or promotions tab. The sending path has moved from JavaScript to Go, while the company is also focusing on lower tail latency and challenging assumptions about what email infrastructure must cost or how it must behave.

Rocha says Resend’s product philosophy is to be “formless”: users should be able to do what they want through APIs rather than waiting for Resend to anticipate every use case. A local MCP server was first built by an investor, then transferred into Resend’s organization after gaining traction. Building a remote MCP required a permissions redesign, OAuth, resource-level access decisions, observability, and distribution through official marketplaces. MCP usage grew from roughly 50% month-over-month growth to doubling monthly, reaching about 1.2 million calls by the end of August. Rocha trusts integrations more when the software vendor officially maintains them; he switched from Plausible to PostHog and chose Strava partly because their official MCP integrations were available.

The host contrasts Resend’s roughly 80 MCP tools with Cloudflare’s smaller search-and-execute approach. Rocha says he builds according to his own use: an investor update was assembled by fetching data through MCP. Resend began with fewer tools but kept expanding toward full dashboard coverage, including usage, billing, invitations, provisioning, and deletion. He admits that exposing every capability is difficult and that MCP clients may receive too much data, but he values reaching the long tail of users—even if only a few hundred of the 4.3 million users need a particular SDK or tool. Resend also supports CLI and API job applications, and Rocha treats CLI applicants as unusually motivated because they install, authenticate, and complete an unfamiliar flow.

Rocha says Resend was far behind established competitors in traditional SEO, so it focused on focused documentation such as “how to use X with Y” pages and then added FAQ sections across the marketing site. It also created llms.txt and Markdown versions of documentation and marketing pages. A pricing experiment showed why: when an agent could not execute a JavaScript pricing calculator, it incorrectly described Resend’s pricing as obscure; after the relevant numbers were written into the Markdown page, the agent answered correctly. Rocha says recent tools benefit from recency bias and from organic discussion, so companies should make accurate information available in formats agents can read, track recommendations, and test changes rather than assuming conventional SEO tactics are sufficient.

Resend supports both source-controlled, programmatic templates and a multiplayer visual editor for teams that want marketing users to edit templates. Rocha says Resend’s own all-hands meetings use a shared email document with roughly 20 people, after which the group sends the email together. He is also exploring agent-accessible inboxes: APIs for creating many inboxes, reading and marking messages, starring, labeling, and otherwise letting agents operate on inbound email. He estimates Gmail accounts for at least 60% of traffic, followed by Outlook and Yahoo, while Apple’s private relay-style addresses are stricter and require special registration help. Reputation has shifted from being mainly IP-based to being strongly tied to domains and subdomains, so transactional, marketing, and confidential mail should be separated.

Rocha moved from San Francisco to spend more time in Brazil while expecting his second child. During YC’s Winter 2023 batch, he compared himself with founders who had fewer family responsibilities and wanted Resend to be the best company in the batch, but later concluded that YC has no permanent winner: companies pivot, break up, or disappear. A partner criticized him for caring too much about design; Rocha accepted that design might slow growth but decided he would rather build a distinctive product used by 1,000 people than a product he did not believe in used by 10,000. He applies the same calculation to family: a $10 million company with a close relationship with his daughter is preferable to a $100 million company whose success costs that relationship.