Example of the WP Biographia WordPress plugin's wp_biographia_link_item filter; appends the current post's title to the Biography Box's mailto link. Copy and paste this code into the bottom of your theme's functions.php.

wp-biographia-mailto-subject-filter.php

add_filter ('wp_biographia_link_item', 'filter_link_item', 10, 2);
function filter_link_item ($content, $params) {
  // $params = array (
	//		'type' => 'link type (icon|text)',
	//		'format' => 'link format string',
	//		'meta' => 'additional anchor attributes',
	//		'title' => 'link title',
	//		'url' => 'link URL',
	//		'body' => 'link body text',
	//		'link-class' => 'link CSS class name',
	//		'item-class' => 'link item CSS class name (icons only)'
	//	);

	$url = $params['url'];
	if (is_single ()) {
		$pos = strpos ($url, 'mailto:');
		if ($pos === 0) {
			global $post;
			$url .= '?subject=' . $post->post_title;
			$content = sprintf ($params['format'], $url, $params['meta'], $params['title'], $params['link-class'], $params['body'], $params['item-class']);
		}
	}
	return $content;
}