Unable to send/receive e-mail between my e-mail server Postfix/Dovecot and a client.

[Cassius V. de Magalhaes]

Hello,

I was able to install and configure an e-mail server using Postfix and 
Dovecot (SMTP, POP3 and IMAP) but it works only locally on the machine 
where it was installed. I'm unable to send/receive messages between the 
server and a client (on another machine).

My references are:
    1) The Book of Postfix, ISBN 1-59327-001-1, edit. No Starch Press;
    2) Postfix: The Definitive Guide, ISBN 0-596-00212-2, edit. O'Reilly;
    3) Ubuntu Server Guide, URL: 
http://doc.ubuntu.com/ubuntu/serverguide/C/postfix.html ;
    4) Howto: ISP-style Email Server with Debian-Etch and Postfix 2.3, 
URL: http://workaround.org/articles/ispmail-etch/

The e-mail server and the client are running on Ubuntu Server 8.04 LTS 
and Ubuntu Desktop 8.04, respectively.

Any ideas, please?


TIA,
Cassius.