docs: update Mailgun configuration with correct sandbox domain

- Update domain to sandbox1835a1917b07443f8c1ca858689c42ea.mailgun.org
- Set test email to psbailey31@gmail.com
- Add comprehensive Mailgun setup guide
- Document sandbox authorization requirements
- Create email testing script
- Update troubleshooting documentation

Note: Sandbox domains require recipient authorization in Mailgun dashboard

NOTIFICATION_SYSTEM_IMPLEMENTATION.md

@@ -101,10 +101,23 @@ TEST_EMAIL="test@example.com"
 
 ### Mailgun Setup
 
-1. **API Key**: Provided - `dd27be7a81dd7b0980ada31fec9a9bd9-e80d8b76-fd80c63b`
-2. **Domain**: Configure your Mailgun domain (currently using sandbox)
-3. **From Email**: Set your sender email address
-4. **Sandbox Mode**: For testing, add authorized recipients in Mailgun dashboard
+1. **API Key**: `dd27be7a81dd7b0980ada31fec9a9bd9-e80d8b76-fd80c63b`
+2. **Domain**: `sandbox1835a1917b07443f8c1ca858689c42ea.mailgun.org`
+3. **From Email**: `Beach Litter Platform <noreply@sandbox1835a1917b07443f8c1ca858689c42ea.mailgun.org>`
+
+#### Sandbox Mode Authorization
+
+**IMPORTANT**: Mailgun sandbox domains require you to authorize recipient email addresses before sending.
+
+To authorize recipients:
+1. Go to https://app.mailgun.com/
+2. Navigate to Sending → Domains
+3. Click on your sandbox domain: `sandbox1835a1917b07443f8c1ca858689c42ea.mailgun.org`
+4. Go to "Authorized Recipients" section
+5. Add email addresses you want to test with (e.g., `psbailey31@gmail.com`)
+6. Recipients will receive a confirmation email - they must click the link to authorize
+
+**For Production**: Use a verified custom domain instead of sandbox to send to any email address without authorization.
 
 ## Testing
 
@@ -218,11 +231,16 @@ Emails include:
 
 ### Emails Not Sending
 
-1. Check Mailgun API key is correct
-2. Verify domain is configured in Mailgun
-3. Check sandbox mode restrictions (add authorized recipients)
-4. Review server logs for error messages
-5. Test email service with test script
+1. **Check Mailgun API key is correct**: Verify in `.env` file
+2. **Verify domain is configured**: Should be `sandbox1835a1917b07443f8c1ca858689c42ea.mailgun.org`
+3. **Authorize recipients for sandbox**: 
+   - Go to https://app.mailgun.com/
+   - Navigate to your sandbox domain
+   - Add authorized recipients (they must confirm via email)
+   - Error "Forbidden" means recipient is not authorized
+4. **Review server logs**: Check for detailed error messages
+5. **Test email service**: Run `npx tsx src/scripts/testEmail.ts`
+6. **For production**: Use a verified custom domain to avoid sandbox restrictions
 
 ### Notifications Not Appearing