I don't use Outlook, but I did find Mail::Outlook. The "all_accounts" method looks promising. So perhaps you could just ask Outlook who it knows about? The aliases may be in the account object. Otherwise, I guess you could build your own alias table. I am not aware of any DB that Net::SMTP would have access to. Keep in mind that the alias may not resolve to a unique email address. Example when I'm sending email, I type in "Tom" and then get a list for me to pick the "right" Tom.

Update: I did a google search export+outlook+address+book. Evidently there is a way to export the address book as a .CSV file. Since I don't have Outlook, I don't know exactly what is in there or what that would mean on a corporate network vs standalone Outlook or Outlook Express. Just an idea that may or may not help.