Email service providers (ESP) let users build email lists and send emails. Without a proper ESP, running an email outreach campaign would be more or less impossible.
Other than storing email addresses and sending emails, an email service provider might come with additional features, such as extra storage, built-in A/B testing, email marketing automation, website tracking, or segmentation based on recipients’ interests.
Despite offering email access, services like Gmail, Outlook, or AOL are not considered proper ESPs when talking about email marketing. The difference between these three and proper email service providers is that the latter lets users utilize email contact lists and send campaigns to subscribers.