An Email service provider, or ESP, is a company that provides Email services to other companies or individuals. An Email service provider can provide personal Email services and/or mass Email services.
An Email service provider that provides personal Email services is a company that handles either personal or company individual Email inboxes. An example of an Email service provider that provides personal Email services is Gmail®.
An Email service provider that provides mass Email services is a company that provides companies or individuals with the ability to Email multiple recipients with the same Email message. Mass Emails are normally sent to hundreds, if not in the thousands, of recipients. These types of Email service providers also provide metrics on the mass Email campaigns that are sent out. Metrics include items such as how many people opened the Email, how many people clicked a link in the Email, etc.