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Smart Emailing
An integration with Czech and Slovak CRM Smart Emailing: https://app.smartemailing.cz/docs/api/v3/index.html#api
It has the best deliverability to Czech and Slovak mailboxes ad that's why majority of our online enterpreneurs choose it.
Michaela shared this idea 
It has all the API endpoints we’d need but it will require some creativity.
For example there aren’t methods specifically for adding contacts to lists or removing them, so we’ll need to first load the contact, edit their lists, and sync them back.
This isn’t great for performance but it’s technically possible. We’ll leave this open to see what the interest is like from other customers.
If you’re interested in taking this on yourself we also have a guide at https://wpfusion.com/documentation/advanced-developer-tutorials/creating-custom-crm-modules/