In the *nix mail paradigm, mail generally arrives at your system via either a Mail Transfer Agent (MTA) such as Sendmail, Exim, or Postfix, or a Mail Retrieval Agent (MRA) such as Fetchmail or Getmail. It is then delivered locally by a Mail Delivery Agent (MDA) such as Procmail or Maildrop (some MTAs and MRAs have at least some built in MDA functionality). There are many variations and possibilities, and general configuration instructions are therefore beyond the scope of this document, but we will provide a basic outline of configuring your mail system for Mailmin.
Mailmin is essentially a specialized MDA; it expects to receive mail on its standard input. The remote system should be configured to deliver mail from one or more accounts to Mailmin. You can configure your MTA or MRA to pass Mailmin mail to Mailmin for delivery; alternatively, you can chain MDAs, configuring an upstream MDA to pass along Mailmin mail to Mailmin.
Using Getmail, for example, you could include the following section in a Getmail configuration file:
[Destination]
type = MDA_external
path = mailmin.pl
[To save a copy of the message to a mailbox, use MultiDestination; see the Getmail configuration guide for more details.]
Note that Mailmin itself does no filtering; it assumes that anything it receives is Mailmin mail. If non-Mailmin mail is expected on the same account, you can use an upstream MDA for filtering. The details of this are beyond the scope of this document.
Mailmin sends its replies via the sendmail command, which today is usually actually a sendmail drop-in such as Exim or Postfix. As long there's such a command on your system, Mailmin should be able to send outgoing mail without any special configuration.