What is an ESP (Email Service Provider)?

ESP (Email Service Provider) is a platform that handles outbound email delivery on your behalf. ESPs operate the actual SMTP infrastructure, manage subscriber lists, handle bounce and complaint feedback loops, provide analytics on opens and clicks, and (importantly) maintain the sender reputation that determines whether your campaigns reach inboxes.

Common ESPs by category:

• Marketing-focused. Mailchimp, ActiveCampaign, ConvertKit, MailerLite, Brevo (formerly Sendinblue), GetResponse, Constant Contact, AWeber, Campaign Monitor, Drip, Moosend, Klaviyo (B2C/e-commerce-focused).
• Transactional-focused. SendGrid, Mailgun, Postmark, Amazon SES, Mailjet, Elastic Email.
• CRM-integrated. HubSpot, Salesforce Marketing Cloud, Pardot.

Most ESPs offer both marketing and transactional sending, though some specialize. The distinction matters because transactional sending typically has lower complaint rates and different deliverability profiles than marketing sending.

Valid Email Checker integrates natively with 17 ESPs. See integrations overview for the full list and connect flow.