diff --git a/krebs/3modules/exim-retiolum.nix b/krebs/3modules/exim-retiolum.nix
index bf37a1ef7..dbd98d059 100644
--- a/krebs/3modules/exim-retiolum.nix
+++ b/krebs/3modules/exim-retiolum.nix
@@ -29,7 +29,7 @@ in {
         # This configuration makes only sense for retiolum-enabled hosts.
         # TODO modular configuration
         assert config.krebs.tinc.retiolum.enable;
-        ''
+        /* exim */ ''
           keep_environment =
 
           primary_hostname = ${cfg.primary_hostname}
diff --git a/krebs/3modules/exim-smarthost.nix b/krebs/3modules/exim-smarthost.nix
index 5f93ae937..e988fb563 100644
--- a/krebs/3modules/exim-smarthost.nix
+++ b/krebs/3modules/exim-smarthost.nix
@@ -121,7 +121,7 @@ let
     };
     krebs.exim = {
       enable = true;
-      config = ''
+      config = /* exim */ ''
         keep_environment =
 
         primary_hostname = ${cfg.primary_hostname}
@@ -233,7 +233,7 @@ let
 
         remote_smtp:
           driver = smtp
-          ${optionalString (cfg.dkim != []) (indent ''
+          ${optionalString (cfg.dkim != []) (indent /* exim */ ''
             dkim_canon = relaxed
             dkim_domain = $sender_address_domain
             dkim_private_key = ''${lookup{$sender_address_domain}lsearch{${lsearch.dkim_private_key}}}
@@ -262,7 +262,7 @@ let
 
         begin rewrite
         begin authenticators
-        ${concatStringsSep "\n" (mapAttrsToList (name: text: ''
+        ${concatStringsSep "\n" (mapAttrsToList (name: text: /* exim */ ''
         ${name}:
           ${indent text}
         '') cfg.authenticators)}
diff --git a/krebs/3modules/exim.nix b/krebs/3modules/exim.nix
index cfcbbc438..83d88cb0d 100644
--- a/krebs/3modules/exim.nix
+++ b/krebs/3modules/exim.nix
@@ -37,7 +37,7 @@ in {
   };
   config = lib.mkIf cfg.enable {
     environment = {
-      etc."exim.conf".source = pkgs.writeEximConfig "exim.conf" ''
+      etc."exim.conf".source = pkgs.writeEximConfig "exim.conf" /* exim */ ''
         exim_user = ${cfg.user.name}
         exim_group = ${cfg.group.name}
         exim_path = /run/wrappers/bin/exim