Transactional Email

The category where deliverability problems are highest-stakes — a marketing campaign in spam annoys subscribers; a password-reset email in spam locks users out of the product. The architectural rule that matters most: transactional and marketing must run on separate IPs and ideally separate domains. Marketing complaints damage reputation; if transactional shares that reputation, the product breaks. This category covers stream isolation, transactional API providers (Postmark, Mailgun, SendGrid, AWS SES), and ecommerce lifecycle email patterns.