diff --git a/TODO.on.site b/TODO.on.site
index 3fbf517..35a9079 100644
--- a/TODO.on.site
+++ b/TODO.on.site
@@ -25,3 +25,10 @@
 * If using a subdomain, set `use_esld = false;`
   in `mail/rspamd-override.d/dkim_signing.conf`
 * Publish key from `mail/config/rspamd/dkim/*.public.dns.txt` via DNS
+
+Mail testing with dial-in IP:
+Some popular (german) email providers greet with a 554 error when the IP is
+blacklisted. Seems out of standard and hence can lead to not bouncing the
+message immediately.
+* Set `smtp_skip_5xx_greeting = no` in `mail/config/postfix-main.cf`,
+  if you want immediate bounces.