diff --git a/modules/sendmail-mta.nix b/modules/sendmail-mta.nix
index 54a2dda..69c99bd 100644
--- a/modules/sendmail-mta.nix
+++ b/modules/sendmail-mta.nix
@@ -17,7 +17,8 @@
   #   sudo smtpctl log verbose
   #   systemctl status opensmtpd
   #   sudo smtpctl show status
-  # Send an email
+  # Send an email.
+  # TODO: One has set the sender explicitely to `-f langbein@mail.de`.
   #   echo "Subject: hello" | sendmail -f langbein@mail.de daniel@systemli.org
   # Queue handling
   #   sudo smtpctl show queue