Ubico lets you blacklist domains to prevent reaching out to current opportunities or existing customers. 

Step 1: Navigate to your account settings.
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Step 2: Add domains to your blacklist by entering it manually or copy-paste it to the text area below. If you're putting more than one domain make sure to separate it with comma, space or new line.
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Once saved, these domains will not be contacted and will be used as a suppression list when searching and collecting data for your contacts.