org-link-mailto-program

org-link-mailto-program is a variable defined in `org.el'.
Its value is

(browse-url "mailto:%a?subject=%s")

• This variable may be risky if used as a file-local variable.

Documentation:
Function and arguments to call for following mailto links.
This is a list with the first element being a Lisp function, and the
remaining elements being arguments to the function. In string arguments,
%a will be replaced by the address, and %s will be replaced by the subject
if one was given like in .

You can customize this variable.