How to connect a Resend account

Resend is a modern, developer-first email API. It's excellent at sending — but like every ESP of its kind, it can't protect you from spam filters on its own. Your sender reputation still depends on how inbox providers perceive your domain, your IP and your sending address.
That's exactly what MailReach does: it warms up your Resend sending address with real, human-like conversations across a network of 30,000+ Google Workspace and Microsoft 365 inboxes, so your emails land in the inbox instead of spam.
This guide walks you through connecting a Resend account to MailReach. If you have everything at hand, it takes about 3 minutes.

Before you start: how a Resend connection works

This is the part most people get wrong, so let's make it crystal clear.
MailReach needs two things to warm an email address:

• The address you warm is, say, flo@yourdomain.com
• Outbound goes through Resend's SMTP
• Inbound lands in a Google Workspace / Microsoft 365 mailbox on the same domain (your MX records point there)

If replies to your Resend address currently go nowhere, you have a deliverability problem far bigger than warmup. Set up a real mailbox first — a sending address that can't receive mail is a red flag for Gmail and Outlook.

Prerequisites checklist

• ✅ A verified domain in Resend (Resend refuses to relay anything from an unverified domain)
• ✅ SPF, DKIM and DMARC properly set on that domain
• ✅ A working inbox for the address you want to warm, with IMAP credentials
• ✅ A MailReach account

Step 1 — Create your Resend API key

Resend uses an API key as the SMTP password. Here's how to get one:

1. Log in to Resend and open API Keys in the left-hand menu.
2. Click Create API Key.
3. Give it a clear name — for example MailReach.
4. Set the permission to Sending access (that's all MailReach needs).
5. Select the domain the key applies to, then click Add.
6. Copy the key immediately. It starts with re_ and Resend only displays it once. If you lose it, you'll have to revoke it and create a new one.

Step 2 — Connect the SMTP in MailReach

1. On your MailReach dashboard, click Connect an account.
2. Choose Any other SMTP.
3. Fill in the sender details:
4. • First name: the first name attached to your Resend sending address
   • Last name: the last name attached to your Resend sending address
   • Email address to warm: your Resend sending address (it must be on a domain verified in Resend)
4. Fill in the SMTP fields:

5. Click Next.

Step 3 — Connect the IMAP inbox

This is where the replies from the MailReach warming network land. It also lets MailReach thread conversations properly and keep the warming emails hidden from your actual inbox.
Enter the IMAP credentials of the mailbox that receives replies to your Resend address:

• Google Workspace / Gmail: you need an App Password (not your regular account password). Follow our guide: Connect a Gmail / Google Workspace to MailReach with an App Password.
• • IMAP host: imap.gmail.com
  • IMAP port: 993
• Microsoft 365 / Outlook: use the Office 365 IMAP settings.
• • IMAP host: outlook.office365.com
  • IMAP port: 993
• Any other provider: check your provider's documentation, or ask their support for the IMAP host, port, username and password.

Click Next. MailReach tests both connections. If both pass, you're done.

Step 4 — Set your warmup settings and start

Once the account is connected:

• Set your starting volume and daily increase. If in doubt, keep the default — MailReach ramps up progressively on its own.
• Run the initial warmup for at least 14 days before sending any campaign from that address.
• Keep the warmup running permanently, including while you send campaigns. Reputation is maintained by a constant engagement rate, not by a one-off warmup sprint.

Then just watch your Reputation Score climb in the dashboard.

A word on Resend's plan limits

Warmup consumes real sends from your Resend quota. Resend's free plan is capped at 100 emails per day and 3,000 per month — MailReach can send up to 100 warming emails per day per address, which will eat that entire allowance on its own.
If you're warming several addresses, or warming and sending campaigns at the same time, plan for a paid Resend plan.

Troubleshooting

"535 Authentication failed" Your SMTP username isn't right. It must be the literal string resend, in lowercase. The password must be the complete API key with its re_ prefix, with no leading or trailing spaces.
"The domain is not verified" Resend refuses to relay any email from a domain that isn't verified in your account. Go to Domains in Resend, add your domain, set the DNS records and wait for verification (usually a few minutes).
"403 / Missing permissions" Your API key doesn't have sending access. Create a new key with at least Sending access.
Rate limit errors (429) Resend caps the send rate per second (roughly 2/sec on the free plan, 10/sec on paid). This is rarely an issue with warmup volumes, but it can trigger if you're sending campaigns at full throttle at the same time.
The warmup sends, but no replies arrive Your IMAP inbox isn't the inbox that actually receives mail for your sending address. Check your domain's MX records and make sure they point to the mailbox you connected in Step 3.
Warming emails show up in my inbox MailReach hides them automatically through IMAP. If they're visible, the IMAP connection is likely broken — reconnect the account.

Updated on: 13/07/2026