E-mail programs depend on scripts that run in your browser, and sometimes those scripts get cranky. Cleaning out the cache of your browser may solve the problem (generally, you'll find it on the "tools" menu). It may also delete cookies for sites you have to log into like Mudcat, so look up the passwords for those Websites.
Cleaning your browser cache can solve or prevent all sorts of problems. I'd recommend doing it once a month or more.