HowTos are usually not provided with your distribution (though some distributions do
provide packages that contain HowTos), but you can find them amass on the web, either distro-specific (for example on the ArchWiki, the SlackDocs project, the Debian Wiki, the Gentoo Wiki, ...) or in places where Linux/Open Source documentation in general is accumulated, like http://tldp.org/
or similar sites. In many cases it is sufficient to add the word HowTo to your search criteria in your search engine of choice, for example searching for Debian mail server HowTo
in DuckDuckGo or Google will give you plenty of relevant hitsfor setting up a mail server on Debian.