The developer's favorite. Unmatched speed and deliverability for transactional email.
Postmark does not want your marketing newsletter. They want your "Password Reset" and "Order Confirmation" emails. By separating these "Transactional" emails from marketing spam, they achieve near-perfect deliverability speeds. Recently acquired by ActiveCampaign.
If you are building an app, use Postmark for the system emails. Use something else for the newsletter.
They have a 'Broadcast' stream, but they are very strict about quality.
Mostly, but they have a UI for template management.