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. Common relays include Amazon SES, Mailgun, Postmark, and SparkPost.

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. Popular ESPs include Mailchimp, Smaily, Klaviyo, and Brevo.

Cost and control differ significantly. Relays charge per email (Amazon SES starts around $0.10 per 1,000 emails) and give you full control over templates and sending logic. ESPs charge per subscriber or per tier, and handle design, compliance, and analytics for you.

Many companies use both: an SMTP relay for transactional mail from their app, and an ESP for marketing campaigns managed by the marketing team. Some providers like SendGrid and Brevo straddle both categories, offering relay infrastructure alongside marketing features.