Wasn't Eudora a little more helpful in explaining what the temporary file was?
So, let's think about why there might be a temporary file that causes errors.
Sometimes a virus scan will hold up or contain a file and that may not be accessible by the client. If you're connecting as a POP3 client then try configuring IMAP but do remember to save / backup any existing downloaded mail messages.
Do you get an error only on multiple file downloads or only on emails with attachments?