Hi there,
I've just configured Postfix and Dovecot on Ubuntu 10.04 following this guide. Everything seems to be working perfectly fine and I've managed to create a user (as explained in the guide) and send/receive a few mails using mutt.
The problem is I don't know how to connect Mail.app or any other desktop mail client. Also I would be grateful if you give me a guide on how to redirect mail to a gmail account too.

Thanks

You should be able to connect using mail.example.com - or whatever you set up your mail domain to be. Use that for incoming and outgoing server in Mail.app. If you haven't allowed the pop and imap ssl ports in your firewall, you'll have to do that as well.