function pum_autoloader( $class ) {

	if ( strncmp( 'PUM_Newsletter_', $class, strlen( 'PUM_Newsletter_' ) ) === 0 && class_exists( 'PUM_MCI' ) && ! empty( PUM_MCI::$VER ) && version_compare( PUM_MCI::$VER, '1.3.0', '<' ) ) {
		return;
	}

	$pum_autoloaders = apply_filters( 'pum_autoloaders', array(
		array(
			'prefix' => 'PUM_',
			'dir'    => dirname( __FILE__ ) . '/classes/',
		),
	) );

	foreach ( $pum_autoloaders as $autoloader ) {
		$autoloader = wp_parse_args( $autoloader, array(
			'prefix'  => 'PUM_',
			'dir'     => dirname( __FILE__ ) . '/classes/',
			'search'  => '_',
			'replace' => '/',
		) );

		// project-specific namespace prefix
		$prefix = $autoloader['prefix'];

		// does the class use the namespace prefix?
		$len = strlen( $prefix );
		if ( strncmp( $prefix, $class, $len ) !== 0 ) {
			// no, move to the next registered autoloader
			continue;
		}

		// get the relative class name
		$relative_class = substr( $class, $len );

		// replace the namespace prefix with the base directory, replace namespace
		// separators with directory separators in the relative class name, append
		// with .php
		$file = $autoloader['dir'] . str_replace( $autoloader['search'], $autoloader['replace'], $relative_class ) . '.php';

		// if the file exists, require it
		if ( file_exists( $file ) ) {
			require_once $file;
		}
	}
}