Mailchimp and Klaviyo directly, from the Email Lists screen, and MailPoet and FluentCRM when either is active on the same site: those keep their subscribers in your own database, so there is no API key and nothing leaves the site. Each new address is handed over in the background moments after the spin, so a slow platform never keeps the wheel waiting. For any other platform, export your leads to CSV from the Leads screen and import them there.
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