[Casimiro Notevi]

Prueba algo así:

Código Delphi [-]
uses
  ComObj, ActiveX, Variants;

procedure GetDefaultAccount;
var
  OutlookApp: OleVariant;
  Accounts: OleVariant;
  Account: OleVariant;
  i: Integer;
begin
  try
    OutlookApp := CreateOleObject('Outlook.Application');
    Accounts := OutlookApp.Session.Accounts;

    for i := 1 to Accounts.Count do
    begin
      Account := Accounts.Item(i);
      if Account.DisplayName = OutlookApp.Session.DefaultStore.DisplayName then
      begin
        ShowMessage('La cuenta predeterminada es: ' + Account.DisplayName);
        Break;
      end;
    end;
  except
    on E:Exception do
      ShowMessage(E.Message);
  end;
end;