Re-download the incomplete mail

Hi everybody,

I have used the app "mail" on my macbook pro for two years. A trouble I met is that the "mail" downloaded the mail from yahoo server or other else server, sometimes some of the important mails for work will be downloaded partly. It's not acceptable and painful for me. I want to re-download the incomplete mail on my "mail", but I can't find out the function for this on my "mail".

Pleas show me the way to get it. Thanks!