[X4U] Remove 'suggested' email addresses from To: line?

[Trade Sniper]

When I start a new email, as I type an address on the To: line, it  
starts trying to guess the email address I'm typing.  There are some  
old email addresses that it suggests that I no longer use and would  
like to delete from its list of possible addresses.  Is there any way  
to do this?  These addresses aren't in my Address Book.

I just deleted the cache and plist files from ~/Library/Caches/Mail  
and ~/Library/Preferences/com.apple.mail.plist, but that didn't make  
any difference.  Where are these suggested addresses stored at?