Mautic

Mautic is an open-source marketing automation platform trusted by over 40,000 organizations worldwide, offering the transparency and customization that comes with open code. You can deploy it on-premises, in a private cloud, or on a Mautic-hosted instance, giving you full control over where customer data lives and how the platform behaves. The source code is available on GitHub, meaning you can audit it, extend it, and modify it to fit your specific agency workflows.

This is especially attractive to agencies serving government entities, healthcare organizations, or other sectors with strict data residency rules that forbid SaaS cloud storage. The learning curve is steeper than click-and-go platforms, and you'll need some technical staff to maintain and customize it, but the long-term cost and flexibility reward agencies making that investment. For multi-client management, Mautic's contact organization and campaign isolation make it workable, though it wasn't designed specifically as a white-label reseller tool.