Customer.io

Customer.io's workflow builder triggers messages from product events rather than list membership, so a user hitting a usage limit, abandoning a signup flow, or going quiet for two weeks can each kick off a different sequence automatically. It supports Liquid templating for developers who want conditional logic inside a message, and the channel list runs past email into SMS, push, in-app, Slack, and raw webhooks for anything without a native integration.

Pricing is based on the number of profiles stored, not the number of emails sent, which matters if your app has far more registered users than active ones. The Essentials plan includes 5,000 profiles and a million emails a month with the full workflow builder, and Premium starts around $1,000 a month for 10,000 profiles with added support and compliance features. There's no permanent free tier, but a startup program grants qualifying early-stage companies up to twelve months free with 30,000 profiles included. It's a good fit for a product-led SaaS or app team that wants event-driven, multi-channel messaging without committing to an enterprise-scale contract on day one.