Mail is downloaded strictly in the order it is received so either allow an extended period for the message to download or log into the web mail at your provider and delete the message to clear the download list so other mail can download.

Also, be sure not to attach any large files to e-mails in future :) They can be a pain if the recipient has no way of deleting the e-mail without downloading it, and has a slow connection. Use a dedicated service to upload the file instead and send only a download link. I recommend Filemail.com for their largest (30GB) free file size limit. They also have a free mobile app, so you can send from your Android / iPhone to your PC. Hope that helps.