Just out of curiosity, there are dozens / hundreds of tutorials helping you setup a dovecot - postfix mail server. On the surface, both of them are email servers, why use both? Mail servers are complicated enough, why have 2 things to manage and troubleshoot?
Makes about as much sense on the surface as running CentOS inside Ubuntu and always wondering why ./configure is confused :)
Dovecot is an open source IMAP and POP3 email server for Linux/UNIX-like systems, written with security primarily in mind. Dovecot is an excellent choice for both small and large installations. It's fast, simple to set up, requires no special administration and it uses very little memory.
What is Postfix? It is Wietse Venema's mail server that started life at IBM research as an alternative to the widely-used Sendmail program. Now at Google, Wietse continues to support Postfix.
I followed one of those many guides and got it all working with TLS authentication to Postfix and ISPConfig, then realized at the end that I really never had to touch Dovecot, but wasn't sure if it had to be there for the spam filters, antivirus or some other thing I am not thinking of.