Mautic

Mautic gives you the full automation builder, landing pages, forms, lead scoring, and multi-channel campaigns of a commercial marketing platform, released as open-source software you install on your own server or through a hosting partner. Acquia, which acquired Mautic in 2019, offers a managed cloud version, but most free users run the community edition on their own infrastructure.

There's no subscriber or send limit built into the software itself, since Mautic doesn't bill by contact count or emails at all, but you're responsible for hosting costs, a separate SMTP provider to actually deliver mail, and the server maintenance a self-hosted tool requires. Managed hosting through third parties or Acquia's cloud version runs from roughly a few hundred dollars a month depending on scale. It fits a technical team or agency that wants full control over its data and unlimited contacts, and doesn't mind trading a monthly subscription fee for server administration.