Email Setup

Cap uses email for passwordless authentication. Without email configured, login links appear in server logs. This guide shows how to set up email delivery using Resend.

Why Email Configuration Matters

Without email:

Users must access server logs to get login links
No password reset emails
No notification emails
Poor user experience

With email:

One-click login from inbox
Professional user experience
Notification support
Production-ready authentication

Resend (Recommended)

Resend is Cap’s officially supported email provider.

Why Resend?

Simple API and setup
Generous free tier (3,000 emails/month)
Excellent deliverability
Built for developers
No credit card required for trial

Setup Steps

Create Resend Account

Go to resend.com
Click Sign Up
Verify your email address
Complete onboarding

Add and Verify Domain

Go to Domains in Resend dashboard
Click Add Domain
Enter your domain (e.g., yourdomain.com)
Add the DNS records shown:

Required DNS Records:

Type: TXT
Name: @
Value: resend-verification=xxxxx

Type: MX
Name: @
Priority: 10
Value: feedback-smtp.us-east-1.amazonses.com

Type: TXT
Name: @
Value: v=spf1 include:amazonses.com ~all

Type: TXT
Name: resend._domainkey
Value: p=MIGfMA0GCSqGSIb3DQEBA...

DNS changes can take up to 48 hours to propagate, but usually complete within minutes.

Verify Domain

After adding DNS records, click Verify in Resend
Wait for verification to complete
Status should show Verified

Verification Failed?

Wait 5-10 minutes and try again
Use DNS checker to verify records propagated
Ensure records match exactly (no extra quotes or spaces)
Check with your DNS provider’s support

Create API Key

Go to API Keys
Click Create API Key
Configure:
- Name: Cap Production
- Permission: Full Access (or Sending Access)
- Domain: Select your verified domain
Copy the API key (starts with re_)

Save the API key immediately. Resend only shows it once.

Configure Cap

Add these environment variables to your .env file:

.env

RESEND_API_KEY=re_xxxxxxxxxxxxxxxxxxxx
RESEND_FROM_DOMAIN=yourdomain.com

The from address will be: noreply@yourdomain.com

Restart Cap

Apply the new configuration:

Docker Compose
Railway
Coolify

docker compose down
docker compose up -d

Test Email

Go to your Cap instance
Click Sign In
Enter your email address
Check your inbox for the login email

If you don’t receive the email:

Check spam folder
Verify domain is verified in Resend
Check Cap logs for errors: docker compose logs cap-web

Using a Subdomain

If you can’t add DNS records to your main domain, use a subdomain:

Create Subdomain

Create a subdomain like mail.yourdomain.com

Add to Resend

Add mail.yourdomain.com as a domain in Resend

Configure Cap

.env

RESEND_FROM_DOMAIN=mail.yourdomain.com

Emails will be sent from: noreply@mail.yourdomain.com

Custom From Address

Change the sender name and email:

This requires code modification. Cap currently uses noreply@ as the sender.

To customize:

Fork the Cap repository
Edit apps/web/app/api/auth/[...nextauth]/route.ts
Find the Resend email configuration

Change from field:

from: `Cap <support@${process.env.RESEND_FROM_DOMAIN}>`

Rebuild and deploy

Email Template Customization

Cap uses default NextAuth email templates. To customize:

Create custom email templates
Modify the NextAuth configuration in apps/web/app/api/auth/[...nextauth]/route.ts
Add custom HTML/text templates

See NextAuth.js Email Provider docs for details.

Alternative Email Providers

While Resend is recommended, Cap can work with other providers using SMTP.

Using SMTP

SMTP support requires code modification. Cap’s current codebase is designed for Resend’s API.

To use SMTP providers (SendGrid, Mailgun, Postmark, etc.):

Modify NextAuth configuration to use SMTP

Update apps/web/app/api/auth/[...nextauth]/route.ts:

import { EmailProvider } from "next-auth/providers/email";

providers: [
  EmailProvider({
    server: {
      host: process.env.SMTP_HOST,
      port: Number(process.env.SMTP_PORT),
      auth: {
        user: process.env.SMTP_USER,
        pass: process.env.SMTP_PASSWORD,
      },
    },
    from: process.env.EMAIL_FROM,
  }),
]

Add environment variables:

.env

SMTP_HOST=smtp.sendgrid.net
SMTP_PORT=587
SMTP_USER=apikey
SMTP_PASSWORD=SG.xxxxxxxxx
EMAIL_FROM=noreply@yourdomain.com

Monitoring Email Delivery

Resend Dashboard

Go to Resend Dashboard
View all sent emails
Check delivery status:
- Delivered - Successfully delivered
- Bounced - Invalid email address
- Complained - Marked as spam
- Queued - Being processed

Cap Logs

Check Cap logs for email sending:

docker compose logs cap-web | grep -i email

Successful email:

Email sent to user@example.com via Resend

Failed email:

Error sending email: Invalid API key

Troubleshooting

No Email Received

Check Spam Folder

Verify Domain

Go to Resend dashboard
Check domain status is Verified
Re-verify if needed

Check API Key

docker compose logs cap-web | grep -i "resend\|email"

Look for authentication errors.

Test Resend API

Test API key directly:

curl -X POST 'https://api.resend.com/emails' \
  -H 'Authorization: Bearer re_xxxxxxxxxxxx' \
  -H 'Content-Type: application/json' \
  -d '{
    "from": "noreply@yourdomain.com",
    "to": "your-email@example.com",
    "subject": "Test Email",
    "html": "<p>Test email from Resend</p>"
  }'

Successful response:

{"id":"49a3999c-0ce1-4ea6-ab68-afcd6dc2e794"}

Emails Go to Spam

Verify SPF, DKIM, DMARC

Ensure all DNS records are properly configured:

SPF: Authorizes Resend to send from your domain
DKIM: Cryptographic signature for authenticity
DMARC: Policy for handling failed authentication

Check with MXToolbox

Warm Up Domain

New domains have poor reputation. Gradually increase sending volume:

Day 1: 10-20 emails
Day 2: 50 emails
Week 1: 100/day
Week 2: 500/day
After month: Full volume

Monitor Bounce Rate

Keep bounce rate below 5%:

Remove invalid email addresses
Use double opt-in for signups
Monitor Resend dashboard for bounces

Improve Content

Avoid spam trigger words
Include unsubscribe link (for newsletters)
Use plain text + HTML version
Proper from name and email

Rate Limiting

Resend free tier limits:

3,000 emails/month
100 emails/day (first month)

If you hit limits:

Check Resend dashboard for current usage
Upgrade to paid plan ($20/month for 50,000 emails)
Or implement email queue to spread sends

Invalid API Key Error

Error: Invalid API key

Fix:

Verify RESEND_API_KEY in .env is correct
Key should start with re_
Ensure no extra spaces or quotes
Create new API key if needed
Restart Cap after changing

Production Best Practices

Security

Never commit API keys to git
Use environment variables only
Rotate API keys periodically
Use separate keys for staging/production
Enable 2FA on Resend account

Deliverability

Always verify your domain
Configure all DNS records (SPF, DKIM, DMARC)
Monitor bounce and complaint rates
Keep email list clean
Use descriptive from names
Include physical address in footer (legal requirement in some regions)

Monitoring

Set up Resend webhooks for delivery events
Monitor email sending in logs
Track bounce and complaint rates
Set up alerts for failed sends
Review Resend analytics monthly

Webhooks (Advanced)

Receive real-time delivery notifications:

Create Webhook Endpoint

Add to your Cap deployment (requires code modification):

// app/api/webhooks/resend/route.ts
export async function POST(req: Request) {
  const event = await req.json();

  switch (event.type) {
    case 'email.delivered':
      console.log('Email delivered:', event.data.email_id);
      break;
    case 'email.bounced':
      console.log('Email bounced:', event.data.email_id);
      break;
  }

  return new Response('OK', { status: 200 });
}

Configure in Resend

Go to Webhooks in Resend
Click Add Webhook
URL: https://cap.yourdomain.com/api/webhooks/resend
Select events: delivered, bounced, complained

Cost Estimation

Resend Pricing:

Tier	Price	Emails/month	Best For
Free	$0	3,000	Testing, small teams
Pro	$20	50,000	Small businesses
Business	$80	250,000	Growing companies
Enterprise	Custom	Custom	Large deployments

Calculating your needs:

Login emails: ~1-3 per user per month
100 users = ~300 emails/month
1,000 users = ~3,000 emails/month (Free tier)
10,000 users = ~30,000 emails/month (Pro tier)

Next Steps

Environment Variables

Complete email configuration reference

SSL/HTTPS

Secure your deployment

Troubleshooting

Debug email issues

Scaling

Scale email for production

Getting Started

Deployment

Configuration

Advanced

Email Setup

Why Email Configuration Matters

Resend (Recommended)

Why Resend?

Setup Steps

Using a Subdomain

Custom From Address

Email Template Customization

Alternative Email Providers

Using SMTP

Monitoring Email Delivery

Resend Dashboard

Cap Logs

Troubleshooting

No Email Received

Emails Go to Spam

Rate Limiting

Invalid API Key Error

Production Best Practices

Security

Deliverability

Monitoring

Webhooks (Advanced)

Cost Estimation

Next Steps

Environment Variables

SSL/HTTPS

Troubleshooting

Scaling

Build docs developers (and LLMs) love

Getting Started

Deployment

Configuration

Advanced

Documentation Index

​Why Email Configuration Matters

​Resend (Recommended)

​Why Resend?

​Setup Steps

​Using a Subdomain

​Custom From Address

​Email Template Customization

​Alternative Email Providers

​Using SMTP

​Monitoring Email Delivery

​Resend Dashboard

​Cap Logs

​Troubleshooting

​No Email Received

​Emails Go to Spam

​Rate Limiting

​Invalid API Key Error

​Production Best Practices

​Security

​Deliverability

​Monitoring

​Webhooks (Advanced)

​Cost Estimation

​Next Steps

Environment Variables

SSL/HTTPS

Troubleshooting

Scaling

Build docs developers (and LLMs) love

Why Email Configuration Matters

Resend (Recommended)

Why Resend?

Setup Steps

Using a Subdomain

Custom From Address

Email Template Customization

Alternative Email Providers

Using SMTP

Monitoring Email Delivery

Resend Dashboard

Cap Logs

Troubleshooting

No Email Received

Emails Go to Spam

Rate Limiting

Invalid API Key Error

Production Best Practices

Security

Deliverability

Monitoring

Webhooks (Advanced)

Cost Estimation

Next Steps