These are my personal notes, I am writing them as I learn about postfix's workings and will update them as I remember.

1.1 What is Postfix?

Postfix is a modular email server designed to be a "drop-in" replacement for sendmail. Postfix is described as modular in that it is made up of several smaller applications, each application is designed to do one task only. E.g. SMTP messages are received by one program to deliver them locally another program is invoked, and to deliver them to another host via SMTP a seperate program is called.

1.2 Why is modularity so important in an email system?

The modular approach taken during the implementation of the Postfix mail system allows individual processes to be replaced to meet the users needs. This is most useful in large enterprise and ISP email environments where custom solutions are required.

The modularity of Postfix also means that the system required less resource overheads than some of the other "monolithic" email servers out there (e.g. Sendmail)