This WordPress plugin will rewrite all relative link and image tags (<a> and <img>) to include a base URL.
RSS feeds require absolute URLs to both validate and function properly.
Feed readers can become confused when seeing relative paths, because they won’t necessarily know what base URL to use. A suggested update to the RSS standard, explained here: Relative links (RSS 2.0 at Harvard Law), has not been widely adopted.
This plugin uses code from Gerd Riesselmann.