What are the major ESPs on the market?

There is no single "best ESP" (Email Service Provider, the platform that actually sends your mail). There are categories, and you pick the one that matches what you are doing. Here is the landscape as of 2026, grouped the way operators actually think about it.

Small to mid-size business marketing. This is the "I have a list, I want to send campaigns and some automations" tier.

• Mailchimp is the default people have heard of. Easy to start, fine for newsletters under 50k contacts, deliverability is acceptable on the shared pool but not special.
• Klaviyo is what e-commerce brands run on, mostly because of the Shopify integration and the segmentation around purchase behaviour.
• ActiveCampaign leans into automation flows and a light CRM. Good if you actually plan to build branching journeys.
• Smaily is a European option (Estonia-based), clean UI, useful if you care about EU data residency for GDPR reasons.
• Kit (formerly ConvertKit, renamed in 2024) is built for creators, newsletters, and paid subscriptions. Strong tagging model.
• Brevo (formerly Sendinblue) is worth knowing because it bundles SMTP and marketing in one account, which suits small teams.

Mid-market and enterprise marketing. This is where you stop paying for software and start paying for a contract with a customer success manager.

• HubSpot if you want email glued to a CRM and your sales team already lives there.
• Braze for cross-channel (email plus push plus in-app), strong on mobile-first apps.
• Iterable is similar territory, growth marketing teams at venture-backed companies.
• Customer.io is behavioural messaging, event-driven, popular with SaaS.
• Salesforce Marketing Cloud (the artist formerly known as ExactTarget) is the big enterprise suite. You pick this when procurement says so, not because the UX won you over.
• Adobe Campaign / Marketo sit in the same bucket if you are already in the Adobe stack.

Transactional and developer-focused sending. Different job. You are sending password resets, receipts, and one-to-one app notifications via API or SMTP, not blasting campaigns. See the difference between a sending API and SMTP submission for how these actually connect.

• SendGrid (Twilio) is the big one, huge volume, API-first.
• Mailgun (Sinch) is the other big one, similar shape.
• Postmark (ActiveCampaign group) is the deliverability nerd's pick. They explicitly separate transactional and broadcast streams so your password reset does not get hurt by your marketing reputation.
• Amazon SES is the cheapest per-email by a wide margin. You get a raw MTA (mail transfer agent), you bring your own warmup, suppression, and reporting. Great if you have engineers, painful if you do not.
• SparkPost (MessageBird) handles very high volume.
• Resend is the newer entrant, popular with React and Next.js teams because of the developer experience.

Hybrid / list cleaning / verification (adjacent, not ESPs but you will see them in the stack). Bouncer, Kickbox, NeverBounce, ZeroBounce on the validation side. And, yes, us at Review My Emails on the deeper hygiene side.

How to actually choose

Ignore brand recognition. Three questions decide it.

1. What kind of mail? Marketing broadcast, lifecycle automation, or transactional one-to-one. Mixing them on the same sending domain is how reputations die. Postmark wrote the clearest piece on why these belong on separate streams.
2. What volume? Under 100k/month, almost any tool works. Over a few million per month, the conversation shifts to dedicated IPs, warmup pace, and whether you need shared vs dedicated infrastructure.
3. Who is operating it? A marketing team wants templates and a UI. An engineering team wants an API, webhooks, and a good bounce and delivery event feed. Pick for the operator, not the buyer.

One more thing nobody tells you. The ESP is not your deliverability strategy. Gmail and Yahoo enforce sender requirements (authentication, low spam rate, one-click unsubscribe) regardless of which logo is on your dashboard. The best ESP will not save a dirty list or a bad send pattern. The worst ESP will not sink a clean program either, just slow you down. Get the basics right first, then pick the tool that fits your job.