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Using Multiple SMTP servers

If your Customer.io subscription is on the Startup plan or higher, you can configure multiple custom SMTP servers to have greater control over your deliverability.

Getting started

First, you will need to configure your primary custom SMTP server. By default your first custom SMTP server will be set to send all your Customer.io email messages.

Single SMTP server

Customizing campaign delivery

To customize the delivery settings by campaign type, you need to add your additional servers in the same manner as you added your first custom SMTP server. Once this is done, your new servers default to sending emails for None.

Additional SMTP servers

This means the new servers will not send any Customer.io generated emails. To change that set them to one of the following options from the drop-down:

  • Segment Triggered: Sends emails for your Segment Triggered campaigns
  • Event Triggered: Sends emails for your Event Triggered campaigns
  • Newsletters: Sends emails for your Newsletter campaigns

It’s mandatory for at least one of your custom SMTP servers to stay set on All Emails. This is the server Customer.io will fall back to for any campaign types that don’t have a specific server specified.

For example, let’s take a look at a scenario with three different servers set:

Example custom SMTP servers

Here the first server is set to All Emails, the second is set to Event Triggered, and the last one is set to Newsletters. In this case, because no server is set to Segment Triggered Customer.io will default to using the first server for segment triggered campaigns since this server uses the fallback All Emails.

Using 4+ servers

We currently only support segmenting email delivery by the three campaign types in Customer.io. This means once you have four or more custom SMTP servers set we only use at most three servers.

If you have at least four servers and set three to specific campaign types we still require a server to be set to All Emails like in this example:

Four custom SMTP servers

In this case, the server set to All Emails isn’t used to send any emails since there is a specific server for each type of campaign. This is necessary so we always have a fallback server in case we add features in the future to make additional options available for delivery customization.

When a server is set to None we stop using that server for email delivery. The campaign type the server previously was set to deliver emails for will now use the server set to All Emails for delivery.

If you have any questions about configuring multiple SMTP servers that weren’t covered here, get in touch with us!