Email delivery problem - Ubuntu Karmic Koala (Ubuntu 9.10) [ISPConfig 2]

Dear Sir/Madam,

I followed Falko Timme's tutorial of "The Perfect Server - Ubuntu Karmic
Koala (Ubuntu 9.10) [ISPConfig 2]" to setup the ISP web hosting system, ISPConfig 2 and Squirrelmail, but I have problem when testing the email (in ISPConfig 2, create client, website, user & mail for squirrelmail etc before testing the email):

1. in Ubuntu prompt, run "mail root" to send email message to my personal
email address [email protected], to email address [email protected] created in ISPConfig 2 for the user & mail. But the email message could neither be received from [email protected] nor [email protected].

2. login into Squirrlemail webmail interface and sent email message to my
personal email address [email protected], but the email message couldn't be received from [email protected] as well.

3. from [email protected], sent email message to [email protected], but the email message couldn't be received in [email protected].

In all, the email message delivery into and out the email system hasn't been
worked so far.

It is much appreciated if anyone can help with this, e.g. the information
guidance to identify and solve the problem; the approach to test each
part/server in the email delivery chain to discover the issue; whatever
helpful approach/method.