SendLit
sendlit.app ↗SendLit is an open-source (AGPL-3.0) email platform. Send newsletters, campaigns and sequences through your own SES or SMTP, or build email into your product with team provisioning and an embeddable editor. Hosted at app.sendlit.app with a free plan; paid plans start at $10/month, and contacts are unlimited on every plan — you pay for the emails you send, not your list size.
Charging for emails sent instead of list size is the right call. Most newsletter tools punish you for keeping a big list of people who rarely open. Does the embeddable editor handle transactional templates too, or is it campaigns only?