Then remove that account and re-add it. Your can easily restore the e-mail from your Time Machine backup, doing so is explained in
Start Mail by clicking its icon in the dock.
Click the Time Machine icon on the dock. The computer will enter Time Machine mode with Mail as the active application.
Use the arrows at the side of the screen to scroll back to a time when the message was in your in-box
Select the message and click "Restore" in the bottom right corner of the screen.
Click "Cancel" in the bottom left to exit Time Machine. Your email message will be stored in a folder named "Time Machine" accessible from the left pane of Apple Mail.
If for some reason you are not using Time Machine to backup then re-enter every preference in the offending e-mail account, one of the preferences may be corrupt even though it may look ok.