If email starts to download, but gets stuck it may be the result of a few things. The email that appears stuck may be very large and will either take a long time to download or will never download; your computer may be short on memory; or, that particular email is corrupt in some way.
Let your computer attempt to download email for about 20 minutes. If no progress has been made, cancel the download process and go to Webmail and view your email. In Webmail you can view and delete the large emails that will not download.