What is an ESP?
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If you're sending more than a few dozen emails a week, you've probably hit the point where your regular email account (Gmail, Outlook, whatever) isn't built for this. That's where an Email Service Provider (ESP) comes in.
An ESP is a company that gives you the tools and infrastructure to send email at scale. They handle the servers, the IP addresses, the authentication setup, and all the technical complexity so you don't have to. Upload a list, build a template, hit send. They take care of the rest.
ESPs fall into a few categories based on what kind of email you're sending:
- Marketing ESPs are built for newsletters, campaigns, and promotional email. Think Mailchimp, Klaviyo, Brevo, ActiveCampaign. They give you list management, segmentation, drag-and-drop editors, and analytics (open rates, click rates, unsubscribe tracking).
- Transactional ESPs handle one-to-one triggered emails like password resets, order confirmations, and shipping updates. Examples: Postmark, SendGrid, Mailgun, Amazon SES. Speed and reliability matter more here than fancy templates.
- All-in-one platforms combine email with CRM, automation, and sales tools. HubSpot is the best-known example.
What an ESP actually does for you: runs the mail servers and manages IP addresses so you're not sending from your laptop. Handles authentication (SPF, DKIM, DMARC) and helps you stay compliant with anti-spam laws (unsubscribe links, suppression lists, bounce processing). Monitors your sender reputation and tells you when something's breaking. Provides analytics so you can see what's working and what's not.
The biggest practical question when choosing an ESP is whether you need marketing email, transactional email, or both. Some platforms handle both but keep the streams separate to protect transactional deliverability from marketing reputation issues. Using one platform for everything without separating your streams is a common mistake that can cause your password reset emails to land in spam after a marketing campaign triggers complaints.
Pricing models vary. Some charge per contact stored (Mailchimp, Klaviyo), some per email sent (Brevo, Amazon SES), and some by feature tier (HubSpot, ActiveCampaign). The right model depends on your list size and how often you send.
And a few ESPs worth knowing about beyond the big names: Mailgun is popular with developers who want API-level control. Customer.io and Iterable focus on behavioral messaging for SaaS and apps. Smaily is built specifically for EU compliance with native GDPR tooling and European data hosting. Elastic Email and SMTP2GO offer affordable alternatives for smaller senders. And Postmark is unique because it refuses to send marketing email at all. It only handles transactional, which keeps its deliverability reputation exceptionally clean.
If you're trying to pick an ESP and you're stuck, we built this almanac partly because every vendor will pitch you on being the best. Book a free call at reviewmyemails.com/sos and we'll walk through what actually matters for your setup.
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