diff --git a/nixos/modules/mail.nix b/nixos/modules/mail.nix
index f1861a2..b8918c7 100644
--- a/nixos/modules/mail.nix
+++ b/nixos/modules/mail.nix
@@ -205,6 +205,9 @@ in {
       files = ["/root/.ssh/known_hosts"]; # for the backup server bragi
     };
 
+    # Update HRZ allowlist
+    # For account details see https://www-cgi.hrz.tu-darmstadt.de/mail/
+    # will stop working if no valid TUIDs are associated to our domain.
     systemd = {
       timers."mailAllowlist" = {
         wantedBy = ["timers.target"];