> ## Documentation Index
> Fetch the complete documentation index at: https://docs.autosend.com/llms.txt
> Use this file to discover all available pages before exploring further.
# How to send emails from Customer.io with SMTP
> Configure Customer.io to send emails through AutoSend SMTP.
export const APP_PATHS = {
home: '/',
quickstart: '/quickstart',
domainConfiguration: '/domain',
apiReference: '/api-reference',
sendEmail: '/api-reference/mails/send',
bulkSendEmail: '/api-reference/mails/bulk',
upsertContactApiRef: '/api-reference/contacts/upsert-contact',
transactional: '/transactional-emails',
emailActivity: '/transactional-emails/email-activity',
emailTemplates: '/transactional-emails/email-templates',
sendingEmail: '/quickstart/email-using-api',
transactionalTroubleshooting: '/transactional-emails/troubleshooting',
marketing: '/marketing-emails',
campaigns: '/marketing-emails/campaigns',
contacts: '/marketing-emails/contacts',
contactsIntroduction: '/marketing-emails/contacts/introduction',
contactsImportCsv: '/marketing-emails/contacts/import-csv',
contactsLists: '/marketing-emails/contacts/lists',
contactsSegments: '/marketing-emails/contacts/segments',
contactsCustomFields: '/marketing-emails/contacts/contact-properties',
contactsContactProperties: '/marketing-emails/contacts/contact-properties',
createContactPropertyApiRef: '/api-reference/contact-properties/create',
listContactPropertiesApiRef: '/api-reference/contact-properties/list',
getContactPropertyApiRef: '/api-reference/contact-properties/get-by-name',
deleteContactPropertyApiRef: '/api-reference/contact-properties/delete',
sender: '/marketing-emails/sender',
unsubscribeGroups: '/others/unsubscribe-groups',
webhookIntroduction: '/others/webhooks/introduction',
webhookEventType: '/others/webhooks/event-type',
webhookRetries: '/others/webhooks/retries',
webhookVerifyRequests: '/others/webhooks/verify-requests',
dynamicTemplates: '/dynamic-templates',
guides: '/guides',
sitemap: '/sitemap.xml',
team: '/others/team',
automations: '/automations',
events: '/automations/events',
sendEventApi: '/api-reference/events/send-event',
smtpIntroduction: '/quickstart/smtp',
betterAuth: '/guides/better-auth',
convexGuide: '/guides/convex',
templateVariables: '/transactional-emails/variables',
suppressions: '/others/suppressions',
rateLimit: '/api-reference/rate-limit',
nodejsSdk: '/sdk/nodejs',
smtpIntegrationGuides: '/guides/smtp',
apiKeys: '/api-keys',
encryptedPayloads: '/others/encrypted-payloads',
apiReferenceIntroduction: '/api-reference/introduction',
lovableGuide: '/ai/integrations/lovable',
aiIntroduction: '/ai/introduction',
aiSkills: '/ai/skills',
aiMcpServer: '/ai/mcp-server',
aiLovable: '/ai/integrations/lovable',
aiBolt: '/ai/integrations/bolt',
aiV0: '/ai/integrations/v0',
aiReplit: '/ai/integrations/replit',
mcpClaude: '/ai/mcp-clients/claude',
mcpCursor: '/ai/mcp-clients/cursor',
mcpCopilot: '/ai/mcp-clients/copilot',
mcpWindsurf: '/ai/mcp-clients/windsurf',
mcpCodex: '/ai/mcp-clients/codex',
mcpAntigravity: '/ai/mcp-clients/antigravity',
mcpChatgpt: '/ai/mcp-clients/chatgpt',
mcpRaycast: '/ai/mcp-clients/raycast',
domainWarmup: '/marketing-emails/domain-warmup',
projects: '/projects',
createAutomationApi: '/api-reference/automations/create-automation',
migrationSendgrid: '/migration/sendgrid',
migrationResend: '/migration/resend',
auth0CustomAction: '/guides/auth0-custom-action',
accountBilling: '/others/account/billing',
accountUsage: '/others/account/usage',
inboundIntroduction: '/inbound/introduction',
listInboundMessagesApi: '/api-reference/inbound-emails/list-messages',
getInboundMessageApi: '/api-reference/inbound-emails/get-message',
downloadInboundAttachmentApi: '/api-reference/inbound-emails/download-attachment',
replyToInboundMessageApi: '/api-reference/inbound-emails/reply-to-message',
wikiDailySendingLimit: '/wiki/daily-sending-limit'
};
export const AUTOSEND_PATHS = {
dashboard: 'https://autosend.com/dashboard',
apiKey: 'https://autosend.com/account/api-key',
faqs: 'https://autosend.com/faq',
marketingEmails: 'https://autosend.com/marketing-emails',
webhooks: 'https://autosend.com/webhooks',
composeByAutoSend: 'https://autosend.com/compose',
emailActivity: 'https://autosend.com/email-activities',
team: 'https://autosend.com/settings/team',
pricing: 'https://autosend.com/pricing',
verifyEmail: 'https://autosend.com/compose/email-builder?template=verify-email',
welcomeEmail: 'https://autosend.com/compose/email-builder?template=welcome-email',
productUpdate: 'https://autosend.com/compose/email-builder?template=product-update',
newsletter: 'https://autosend.com/compose/email-builder?template=newsletter',
automations: 'https://autosend.com/automations',
globalSuppressions: 'https://autosend.com/suppressions/global',
signup: 'https://autosend.com/signup',
domains: 'https://autosend.com/settings/domains',
smtpKey: 'https://autosend.com/settings/smtp',
logoKit: 'https://asend.email/logo',
contactsPage: 'https://autosend.com/contacts/list-and-segments',
accountBilling: 'https://autosend.com/account/billing',
accountUsage: 'https://autosend.com/account/usage',
terms: 'https://autosend.com/legal/terms',
inbound: 'https://autosend.com/email-activities/inbound'
};
Customer.io is a messaging platform for marketing, transactional emails, push notifications, and in-app messages. By default, Customer.io uses its built-in email service, but you can configure it to use AutoSend SMTP for better deliverability and email tracking.
## Prerequisites
Make sure you have a verified domain added in AutoSend to send emails from.
Create a project-specific SMTP key from the SMTP tab in Project Settings.
## Configuration
1. Log in to your [Customer.io Dashboard](https://fly.customer.io/)
2. Go to **Settings** > **Workspace Settings**
3. Click on **Email**
1. Click on **Custom SMTP**
2. Select **Add Custom SMTP Server**
Select **Other SMTP** as your provider type and click **Continue to set up**.
Fill in the following settings:
| Field | Value |
| ------------------ | --------------------------------- |
| **Address** | `smtp.autosend.com` |
| **Port** | `587` (or `465` for implicit TLS) |
| **Authentication** | Select `Plain` |
| **Username** | `autosend` |
| **Password** | Your AutoSend SMTP key (AS\_xxx) |
The SMTP password is no longer your API key. Create a project-specific SMTP key from the SMTP tab in Project Settings.
Customer.io does not support port 25. Use port 587, 465, or 2525 instead.
Click **Finish set up** to save your configuration.
Before sending emails, add a sending domain in Customer.io that matches your verified domain in AutoSend.
1. Go to **Settings** → **Workspace Settings** → **Email**
2. Under **Sending Domains**, add your domain (e.g., `yourdomain.com`)
Create a test campaign or broadcast to verify your SMTP configuration is working correctly.
You can also check the Email Activity dashboard to verify the email was sent through AutoSend.
## Troubleshooting
* Verify your SMTP credentials are correct - Check that your sender email domain is verified in
AutoSend - Ensure you've added a sending domain in Customer.io - Check Customer.io's delivery
logs for SMTP connection errors
* Double-check your SMTP key is correct - Ensure you're using `autosend` as the username - Verify
your SMTP key is active in the SMTP Settings
* Verify the server address is `smtp.autosend.com` - Try using port `465` or `2525` if port
`587` fails - Remember that port 25 is not supported by Customer.io
* Ensure your domain has proper DNS records (SPF, DKIM, DMARC) - Use a professional sender name
and address - Check your domain reputation in AutoSend
## Next Steps
Set up SPF, DKIM, and DMARC for better deliverability
Monitor your email delivery and engagement
Set up webhooks to track email events in real-time
Learn more about AutoSend SMTP configuration