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

Here's a scenario that plays out all the time. A developer needs to send password reset emails from an app, and a marketing manager needs to run a welcome series for new signups. They're both sending email. But they probably shouldn't be using the same tool.

The core split is about who's sending and what they're sending. SMTP relays are infrastructure. You connect your app or server to the relay via code, and it handles delivery. There's no drag-and-drop template builder, no campaign dashboard, no subscriber management. Just a reliable pipe that moves your email from your system to the inbox. Amazon SES, Mailgun, and Postmark are classic examples.

ESPs (Email Service Providers) are platforms built for marketers. They give you visual editors, automation workflows, subscriber lists, segmentation, A/B testing, and reporting. Your marketing team can build and send a campaign without writing a single line of code. Mailchimp, Klaviyo, Smaily, and Brevo all fall here.

So when does each one fit?

You'd reach for an SMTP relay when your app generates emails programmatically (receipts, alerts, password resets), when you have developers managing sending logic and templates in code, when you're dealing with high-volume transactional sending, or when you want pay-per-email pricing and don't need marketing bells and whistles. Amazon SES starts around $0.10 per 1,000 emails. That's hard to beat for pure volume.

You'd reach for an ESP when a marketing or content team needs to create campaigns without developer support, when you need automation like welcome series or re-engagement flows, when subscriber preference management and segmentation matter, or when you want built-in analytics and compliance tooling.

The good news is you don't always have to pick one. Many companies run both: an SMTP relay for transactional email from the app, and an ESP for marketing campaigns. That separation is actually healthy for deliverability. (If your marketing email starts generating complaints, you don't want that reputation bleeding into your password reset emails.)

Some platforms straddle both categories. Twilio SendGrid, SparkPost, and Brevo offer relay infrastructure alongside marketing features. That can work well, but keep your sending streams separate inside the platform so they don't cross-contaminate each other's reputation.

Not sure what setup fits your situation? The SOS hotline is free and there's no pitch waiting at the end.