Debian Configuration :: Optimal Setup For A Mail Server?

I would like to discuss setting up a mail server and its implications and alternatives. First, let us see if I have understood this correctly: A mail server consists of many different components. First, a server to listen to any mail inbound for a specific domain (say postfix), and then a POP3/IMAP server (say dovecot). Then, I should somehow configure the rules by which all mail is forwarded to their respective owners. This should be fairly simple by using debians package managers and dselect or whatever program it is that sets up right packages by use cases at the install.

But now lets assume a more complicated environment, where there are multiple users with different domains and needs. First, we need to send mail to ourselves from webapps for instance for backup purposes. So let's say we have a domain called domain.com setup, and we want to send mail to backup@domain.com. Unfortunately, some configuration issue makes the application get confused, because it is trying to send mail to itself, but doesn't quite understand what it should do. How can this problem can be solved?

Second, how could I configure different domains with different rules. For instance, if I want one domain to have a catch-all account, where random email sent to erroneous accounts is captured? Or if I want to create accounts which are not based on actual Debian accounts, but instead just random usernames (say, danny@domain.com, mike@domain.com, support@domain.com etc.)?

Finally, which are the best web-guis for doing such configuration? What if the customer wants to himself add accounts? I cannot require him to edit text files - especially if he can thus break the whole configuration for other customers as well.. postfix-admin is one, but it is quite crude-looking. Is there something which integrates both postfix, apache and dovecot configuration? How about Webmin?