diff --git a/README.md b/README.md
index 76f0001..6aa75eb 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,8 @@ If something does not work, check the log file at `nextcloud/data/nextcloud.log`
 
 **⚠⚠ Warning:** If you are using more than one backend or especially one backend more often than once, make sure that you still have resp. get unique `uid`s in the database. ⚠⚠
 
+**⚠⚠ Warning:** If you are using tools like fail2ban (https://www.fail2ban.org) to protect your authentication source (e.g. IMAP server), be sure to disable it for the host that runs `user_external`. Otherwise a single user failing to login too many times can practically DoS your whole Nextcloud installation because `fail2ban` will then block the Nextcloud IP address. ⚠⚠
+Instead, install appropriate protection apps within Nextcloud that bans repeated failed login attempts.
 
 FTP
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