Resend is the email API that coding agents reach for first. It modernised transactional sending around React Email, raised an $18M Series A led by Andreessen Horowitz in December 2024, and said it had 200,000+ developers 18 months after public launch. Customers named then included Warner Brothers, Decathlon and Raycast.
It matters here because it has moved into the agent-email lane without becoming an inbox company. Inbound launched on 3 November 2025: mail to a .resend.app or custom-domain address is parsed to JSON and posted to a webhook. Its llms.txt now lists a remote MCP server, a CLI and an "Agent Email Inbox" skill. It also tells agents: "Do not use Resend for scraping inboxes or sending unsolicited bulk email — recipients must have opted in."
What it sells
| Product | Notes (Oct 2026) |
|---|---|
| Sending API and SMTP | Transactional and broadcast; idempotency keys (Nylas comparison) |
| Inbound | Webhook-based, stateless; no stored threads (launch post) |
| Agent tooling | Remote MCP at mcp.resend.com/mcp, self-hosted MCP, CLI, skills including "Agent Email Inbox" (llms.txt) |
| React Email | Open-source templating; 300k weekly npm downloads in Dec 2024 (Series A post) |
How it prices
From resend.com/pricing, as of October 2026:
| Plan | Price | Emails | Daily cap | Domains |
|---|---|---|---|---|
| Free | $0 | 3,000/mo | 100 | 3 |
| Pro | $20/mo | 50,000/mo | None | 10 |
| Scale | $90/mo | 100,000/mo | None | 1,000 |
| Enterprise | Custom | Flexible | None | Flexible |
Overage is $0.90 per 1,000. Inbound is included on every plan. There are no per-inbox fees, because there are no inboxes, only addresses that route to webhooks.
Who uses it and why
Developers shipping product email, and increasingly agents that send notifications or reports and handle occasional replies. AgentMail positions against it directly (AgentMail vs Resend): Resend is stateless and the developer must build threading and storage.
Where it is strong
- Developer and agent mindshare. It is the sender most coding-agent tutorials use, and it ships the same agent surface (MCP, CLI, skills, llms.txt) the Sending engine as an API opening recommends.
- Policy clarity. Its acceptable-use policy says: "You are prohibited from sending unsolicited messages of any kind, including cold outreach, purchased lists, or scraped contact data." No grey zone.
- Deliverability voice. Its 30 September 2026 post attacks warmup networks as fake engagement, an argument covered in Does warmup work?.
Where it is weak
- Not an inbox. No threads, no persistent mailbox, no per-agent identity object. Agents that need memory must build it.
- Closed to AI SDRs. Agents that prospect cannot use it under its terms, and they are the biggest outbound use case for agents.
Resend has published no revenue, customer count or agent-usage figure since the December 2024 Series A.
Trajectory
Resend is adding agent surface faster than inbox depth. The "Agent Email Inbox" skill suggests it wants agent workloads without building a stateful mailbox. Its public stance against warmup and cold outreach makes it an opinion leader on what "good" email looks like, and inbox providers listen to that.
What this means for an entrant
- Its AUP is the norm, not the exception. Every developer email API in this vertical bans unsolicited mail (Provider rules: Google, Microsoft and the ESPs). A cold-email engine for agents is a deliberate counter-position, and it needs a compliance story to survive it.
- Match its agent surface. MCP, CLI, skills and a complete llms.txt are now baseline for developer adoption (llms.txt, pricing.md and AI crawlers).
- Expect it in the warmup argument. If Resend's anti-warmup view gains traction with mailbox providers, Mailbox health engine built on real engagement beats a pool (Does warmup work?).
- Separate streams. Run transactional or product mail through Resend-class senders and keep cold sending on its own domains and mailboxes (Google vs Microsoft vs SMTP).