Postfix should work fine, automatically with Drupal. And you definitely need to make sure you have a working MTA for all sorts of server admin tasks.
But you might also consider using the PHPMailer module for Drupal.
http://drupal.org/project/phpmailer PHPMailer is a separate PHP library which will let Drupal connect securely to your gmail account and use gmail's servers to send your mail.
Advantages include saving copies of sent messages on Google's servers, use of both SPF and (since last week) DKIM for anti-spam so your emails don't get flagged as spam, and your emails look like they come from your domain and not the RDNS or hostname of the server.
Major disadvantage is that Google only allows 500 emails per day.