apply_filters ( 'wp_sharing_email_send_post_subject', string $var )

Notes

You can use this filter to customize the subject line of the emails that are sent via the Email sharing button. Here is an example:

function jeherve_email_sharing_customize_subject() {
	return 'Check this great post I just found!';
}
add_filter( 'wp_sharing_email_send_post_subject', 'jeherve_email_sharing_customize_subject' );