What does a "Unknown system type" error mean when attempting to compile Postfix 2.11 from source?
When trying to compile Postfix 2.11 from source on a Linode that is running the Linux 5.4.10-x86_64-linode132 kernel, I get the following error message:
(echo "# Do not edit -- this file documents how Postfix was built for your machine."; /bin/sh makedefs) >makedefs.tmp ATTENTION: ATTENTION: Unknown system type: Linux 5.4.10-x86_64-linode132 ATTENTION:
What does it mean and how can I fix it?
Judging from the output you provided, it looks like your system is having trouble compiling or configuring the compiler with your current kernel version.
After some research, I found a forum thread with someone describing almost the exact same error as you are having
It appears that older versions of postfix (such as 2.11) do not have support for more up-to-date kernel versions and that is likely why you are seeing that error. Looking at Postfix's release page, it looks like version 3.5 might be their latest stable release
You can download newer versions of their source from one of their mirrors here
They also have a handy installation from source code guide with instructions on how to compile from source.