Email service provider
A platform (Mailchimp, Klaviyo, SendGrid, Resend, etc.) that sends, tracks, and manages email at scale.
An email service provider (ESP) is a platform that handles the technical and operational work of sending email at scale: list management, sending infrastructure, deliverability monitoring, template editing, automation, and analytics. Common ESPs include Mailchimp, Klaviyo, ActiveCampaign, ConvertKit, SendGrid, Postmark, and Resend.
ESPs split broadly into two categories: marketing ESPs (Klaviyo, Mailchimp, ConvertKit) optimized for campaigns, automations, and segmentation; and transactional ESPs (SendGrid, Postmark, Resend) optimized for high-deliverability per-user emails like password resets and receipts. Many production email programs use both, on different domains, to isolate reputation.
ESP choice has large downstream effects. Marketing ESPs typically charge by contact count; transactional ESPs charge per email. Migration between ESPs is non-trivial — list segments, automations, and templates rarely port cleanly. Choose carefully.