Now that I'm home and using OE, I checked my machine. I hate to tell you this, but nnaxor's right, even though that article doesn't refer to OE6.
Try this - when looking at the mail accounts tab, click the "?" near the X (close) button, then click on the grayed out 'sort order'. The result: Click to set the order in which directory services are searched when validating e-mail addresses
Go to the directory services tab and you will find the button enabled. Apparently, it doesn't apply to mail accounts.