According to GDRP regulations, in order to send email marketing campaigns to your customers, they need to give their explicit consent to receive such campaigns. 

The best way to collect this consent is during the checkout process or during the registration with a simple opt-in checkbox. This checkbox cannot be pre-checked and should clearly say what they're agreeing to: "I wish to receive email marketing messages."

For Shopify, Magento and WooCommerce, any popup or form can be integrated with Metrilo to automatically sync the permissions given by the user so you won't accidentally send an email to someone who didn't opt in.

Fortunately, Shopify made the collection of email marketing consent super easy. You just need to make sure your store promotional email checkbox is turned on and the settings are correct:

In your Shopify admin panel, go to Settings > Checkout and select "By default, customer doesn't agree to receive promotional emails". This is a must if you sell in the EU because everything's opt-in in Europe unlike the US.
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​Metrilo syncs automatically with the subscription status of the user so you will be able to send follow-ups and emails only to the users who opted in.

WooCommerce will soon introduce such opt-in checkbox option and we will integrate with it for automatic sync. 

Until that time you can use plugins to collect consent. For example, WPGDPR  can do that for you. Here is how to integrate any plugin that you choose to use with Metrilo.

Similar to WooCommerce, Magento store owners can choose which plugin to use for collecting email marketing consent. Here is how to integrate any plugin that you choose with Metrilo.

How to make my pop-ups and forms GDPR compliant?