Definition - What does Email Handler mean?
An email handler is a program that is run when a user clicks on an email link. It is the default or configured email client or the program associated with email links.
Techopedia explains Email Handler
An email handler is configured by default by the operating system, browser or any installed email client application. It is run when a user clicks an email link/address on a Web page or in a document. In this case, the email client is in compose mode and the value of the “To” field is the email address that was clicked.