It sounds like you're setting up a local development environment on your laptop for Drupal.
If this is true, then there's a couple of ways you can go about this (note: I don't use Ubuntu Desktop very much besides Web surfing, so there may be point-and-click ways to achieve the following):
send all your test emails to a local account. You need to configure Postfix as 'local only' (if you run
sudo dpkg-reconfigure postfix it'll ask you again how you want to configure your mail system. Choose local only, and choose your hostname.local or whatever as your mail domain) and then you can use
mutt as a simple mail client through Terminal.
This is probably the easiest and again, if you're doing dev work, you don't have to worry about sending mail out to real addresses by mistake, SMTP blocking on home connections, or even an Internet connection for that matter.
If you really need to send to Internet email addresses, you'll have better luck setting up gmail as your smarthost and reconfiguring Postfix as "Satellite with Smarthost" in the
dpkg-reconfigure postfix setup. There's a bit more to it, but this has been covered quite a bit on ServerFault. By using gmail as your smarthost on an alternative port (tcp 587), you can get around ISP firewalls blocking SMTP and should be able to send email out of most Internet connections.
As for Drupal, it's been a while, but I believe by default (and as with most PHP applications), it'll use whatever is configured in the
sendmail_path setting, but by default, it'll use Postfix's built-in
sendmail wrapper on the localhost (your laptop) so you shouldn't need to change anything there.
Perhaps someone else can clarify that and I'll update my answer accordingly.