How the Resend integration works

Connecting a domain takes three inputs: a domain verified in your Resend account, a Resend API key, and a list of recipient inboxes you own or control. Once you paste the key in, every warm-up email goes out through your own Resend account — not through ours. That single design decision drives everything on this page: your quota gets used, your history gets built, and Resend's rules apply to you directly.

Setting it up

1. Verify your domain in Resend. Add the DNS records Resend generates (SPF, DKIM, and a Return-Path record). Per Resend's docs, verification usually completes within 15 minutes, though DNS propagation can take up to 72 hours. This step is not optional — Resend rejects sends from unverified domains with a "domain is not verified" error, and a valid API key alone does not get around it.
2. Create an API key in the Resend dashboard.
3. Add the domain in WarmEnvelopes and paste the key. Each domain stores its own key, and you can run up to 10 domains.
4. List the recipient inboxes you own, then press Start.

From that point a scheduler sends a slowly increasing number of real emails each day from your domain to those inboxes, spread across the day rather than dumped in a burst — sudden spikes are exactly what Resend's warm-up guide tells senders to avoid, to the point of publishing hourly caps. On the free plan the volume is fixed at 4 emails per day per domain; on Pro you configure the starting volume, the daily increase, and the ceiling — plan details are on the pricing page. Every send is logged with its sender, template, and delivery status.

It counts against your Resend quota — by design

Because the mail leaves through your key, every warm-up send counts against your Resend plan. As of August 2026, Resend's free tier is capped at 3,000 emails per month and 100 emails per day — both limits apply, and everything on the account shares them, including your application's transactional mail and any inbound email you receive through Resend. The WarmEnvelopes free plan's 4 emails a day (~120 a month) fits inside that with room to spare; a Pro ramp heading toward hundreds of emails a day will hit the 100/day cap and needs a paid Resend plan (from $20/month for 50,000 emails as of August 2026, per Resend's pricing). If you're weighing the total cost of warming up, our guide to free email warmup walks through the arithmetic.

Why is spending your quota a feature rather than a bug? Because reputation history accrues where mail actually originates. Your warm-up mail is signed with the same DKIM keys, leaves through the same infrastructure, and appears in the same Resend logs as the production email you're warming the domain for. Warming through someone else's account or a pooled sender would build history somewhere you'll never send from. It also means there is nothing to migrate or unwind: stop the warm-up and nothing about your sending setup changes.

"Resend says you don't need a warmup service"

It does — and you should read it. Resend published Do You Need a Warmup Service? in January 2026, concluding that legitimate senders don't need one. Look closely at what the post actually criticizes: services that control both the sending accounts and the receiving inboxes, generating synthetic opens and scripted auto-replies inside a closed loop of artificially connected inboxes, with pools that can contain reclaimed addresses behaving like spam traps. Every one of those objections describes the peer-network model. None of them describes sending your own mail to your own inboxes.

Meanwhile, the same post's "Real Signals" column lists "gradual, consistent volume increases" as a legitimate reputation input, and Resend separately publishes a day-by-day warm-up schedule it expects new senders to execute by hand — starting around 150 emails on day one and reaching 2,000 a day within a week for a new domain, with hourly maximums along the way. That ramp is precisely the thing WarmEnvelopes automates. Our claim is deliberately narrow: we do not manufacture reputation and cannot guarantee inbox placement. We execute the gradual, consistent volume increase that Resend's own guidance calls for, and we log every send so you can see it happening.

What about the acceptable use policy?

Resend's Acceptable Use Policy (last updated August 5, 2026) prohibits unsolicited messages of any kind — explicitly including cold outreach, purchased lists, and scraped contact data — and requires that all mail go to recipients who have explicitly opted in. Warm-up mail to inboxes you own is none of those things: sender and recipient are the same party, so consent is inherent. It also keeps you clear of the AUP's behavioral triggers — accounts can be suspended above a 4% bounce rate or a 0.08% complaint rate, and mail to your own inboxes essentially never bounces and never gets marked as spam.

Two honest caveats. First, the AUP governs all of your sending: warming up a domain does not make cold outreach acceptable afterward — Resend bans it outright, warmed domain or not, so your real campaigns must qualify on their own. Second, Resend's AUP contains sole-discretion clauses, and Resend clearly dislikes the warmup category as a whole. We found no case of Resend acting against anyone for sending to their own inboxes, but we won't tell you the risk is zero. What we can tell you is that, as of August 2026, nothing in the policy prohibits it — and that everything we send on your behalf is mail you could have sent yourself from a script.