Postmark used to be great for transactional email, but since ActiveCampaign bought them, things have gone downhill compared to Customer.io.
Only use Customer.io if you want to build segmented onboarding flows or send updates to tools like Slack, but skip it for full-featured email marketing.
Customer.io stands out for managing complex, segmented onboarding funnels and giving you a visual way to control communication flows. If you need to build multi-step onboarding that responds to user actions, Customer.io is the obvious pick. It also plays well with tools like Slack, so updates and event pushes are simple to automate.
Postmark used to be strong for transactional emails like password resets, but since being acquired, its quality has dropped off. There's no mention of advanced segmentation or workflow tools, so it's not built for the same kind of communication flows.
If you care about onboarding journeys and need flexibility with integrations, Customer.io is the way to go. Postmark only makes sense if you're after basic transactional emails, but even then, its recent decline makes it hard to recommend over Customer.io for anything beyond the simplest needs.
