When would I need an SMTP relay vs. an ESP?

The choice depends on what you're sending and how much control you need. SMTP relays are ideal for transactional email (password resets, receipts, notifications) and developer-driven sending where you build everything yourself. ESPs (Email Service Providers) add marketing features: visual template builders, subscriber list management, automation workflows, and campaign analytics.

Choose an SMTP relay when: your application generates emails programmatically, you have developers who can build templates and manage sending logic, you need high-volume transactional delivery, or you want to keep costs low with pay-per-email pricing.

Choose an ESP when: marketing teams need to create and send campaigns without developer help, you want built-in automation (welcome series, abandoned cart), you need subscriber management with preference centers and segmentation, or you want detailed campaign reporting and A/B testing tools.

Many companies use both: an SMTP relay for transactional mail from their app, and an ESP for marketing campaigns managed by the marketing team.