diff --git a/pkgs/servers/mail/postfix/default.nix b/pkgs/servers/mail/postfix/default.nix
index 5955789d566f..e9dea5282aa9 100644
--- a/pkgs/servers/mail/postfix/default.nix
+++ b/pkgs/servers/mail/postfix/default.nix
@@ -99,6 +99,11 @@ in stdenv.mkDerivation rec {
       --prefix PATH ":" ${lib.makeBinPath [ coreutils findutils gnugrep ]}
     wrapProgram $out/libexec/postfix/postfix-script \
       --prefix PATH ":" ${lib.makeBinPath [ coreutils findutils gnugrep gawk gnused ]}
+
+    # Avoid dev-only outputs from being retained in final closure.
+    # `makedefs.out` is a documenttation-only file. It should be safe
+    # to store invalid store paths there.
+    sed -e "s|$NIX_STORE/[a-z0-9]\{32\}-|$NIX_STORE/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-|g" -i $out/etc/postfix/makedefs.out
   '';
 
   passthru = {