I run my on mail server and I use Postfix/Dovecot/SquirrelMail/ThunderbirdMail. I know that Postfix is SMTP server and Dovecot is IMAP/POP3 server. Could you explain to me in simple English why Postfix isn't enough. What exactly each server does? How they interact together? Why there's no just ONE mail server that could do it all alone?