Optional: Send Email Through Your Own SMTP Server

Optional: Send Email Through Your Own SMTP Server

By default, Customer.io takes care of your email delivery. However, you can optionally tell Customer.io to send mail through any other SMTP server.

Upsides to doing this

  • You're in full control.
  • We won't charge you anything based on the volume of email you send. You'll only pay the plan cost based on the number of profiles in your account.
  • We'll still handle your open and click tracking.
  • We'll keep a full copy of your sent emails associated with the recipient.

Downsides to doing this

  • Customer.io won't have a feedback loop for data from the sending ESP.
  • Data about delivered, bounced and spammed emails won't be visible in Customer.io. We have integrations providing delivered, spam and bounce data with Mailgun, Mailjet, Mandrill, Sendgrid and Sparkpost

(Optional) Whitelist our IP addresses

If your SMTP server uses an IP whitelist to only allow certain IP addresses to connect you'll need to whitelist Customer.io IP addresses prior to configuring custom SMTP in the application.

The IP addresses that must be whitelisted are as follows:

Note: This list is subject to change as we grow and scale our infrastructure over time. Any additions or removals will be communicated to current Custom SMTP users at least 7 days in advance of the change going into effect.

Configure custom SMTP settings

  1. Head to Workspace Settings -> Email -> Custom SMTP Settings for your selected workspace
  2. Click the Add Custom SMTP Server button, and then fill out the form:

Adding custom SMTP settings

Adding one custom SMTP server

When you save these settings, we'll start sending emails through your own SMTP account.


  • Due to a limitation with our cloud provider, you cannot send SMTP messages using port 25; major SMTP providers should allow delivery using port 465, 587 or 2525 (like in our screenshot above). If you need alternatives, the best thing to do is consult your delivery provider's documentation for ports that work with their service. Regardless of port used, we always initiate a TLS connection (via the STARTTLS command, when supported by your provider), to ensure the security of your outbound messages.

  • If you enabled custom SMTP, you are still required to complete the authentication process. The SPF and DKIM records need to be set up for your chosen STMP. Inside Customer.io, you need to verify the domain and the CNAME record if you want the tracking links to be branded using your domain rather than customeriomail.com.

  • If your Customer.io subscription is on the Premium plan, you can add additional SMTP servers to segment deliveries by campaign type. For more details check out our documentation on configuring multiple SMTP accounts.

If you have any questions about setting this up, get in touch!

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