diff --git a/docs/notifications.md b/docs/notifications.md
index 5741566..22da93e 100644
--- a/docs/notifications.md
+++ b/docs/notifications.md
@@ -27,6 +27,7 @@ To receive notifications by email, the following command-line options, or their
 - `--notification-email-server-user` (env. `WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER`): The username to authenticate with the SMTP server with.
 - `--notification-email-server-password` (env. `WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD`): The password to authenticate with the SMTP server with.
 - `--notification-email-delay` (env. `WATCHTOWER_NOTIFICATION_EMAIL_DELAY`): Delay before sending notifications expressed in seconds.
+- `--notification-email-subjecttag` (env. `WATCHTOWER_NOTIFICATION_EMAIL_SUBJECTTAG`): Prefix to include in the subject tag. Useful when running multiple watchtowers.
 
 Example: