The real answer is that the installer is demanding something that isn't needed by qmail, or indeed by smtp. But, the alternative, if you don't want to buy or fake an fqdn (you could tell it you were foo.amazon.com if you wanted) is either to install from source (eg using life with qmail) or use something like qmail-rocks. The problem you'll then get is that when you distribution upgrade your system it's likely to break your qmail installation. So, pressurise the BOFH wannabe package maintainer to take out this silly requirement. And while they're at it, they could offer something useful in the kind of way exim installation does, such as asking if you want to use a smarthost (you probably do if you're installing on a machine without an fqdn, and you need to specify it in qmail's control file smtproutes).