inc/class-Iac_Mail_ID.php
public static function generate_ID( $type, $item ) {

		$item = self::sanitize_item( $item );
		$type = in_array( $type, array( 'comment', 'post' ) )
			? $type
			: 'post';
		$blog_ID = get_current_blog_ID();
		$base = array(
			$type,
			$blog_ID,
			$item->ID,
			$item->date_gmt
		);
		$url_parts = parse_url( get_option( 'siteurl' ) );

		$ID = sha1( implode( ':', $base ) ) . '@' . $url_parts[ 'host' ];

		return $ID;
	}
public static function generate_ID( $type, $item ) {

		$item = self::sanitize_item( $item );
		$type = in_array( $type, array( 'comment', 'post' ) )
			? $type
			: 'post';
		$blog_ID = get_current_blog_ID();
		$base = array(
			$type,
			$blog_ID,
			$item->ID,
			$item->date_gmt
		);
		$url_parts = parse_url( get_option( 'siteurl' ) );

		$ID = sha1( implode( ':', $base ) ) . '@' . $url_parts[ 'host' ];

		return $ID;
	}
generates the ID based on content type (comment or post)

@param string $type
@param object $item