added note about Postfix check_policy_service limitations with recipient reporting

Author: onlime

README.md

@@ -157,6 +157,12 @@ Make sure to reload Postfix after this change:
 $ systemctl reload postfix
 ```
 
+> [!IMPORTANT]
+> In Postfix, when multiple recipients are specified in the `To:` field of an email, the Postfix policy delegation protocol (the `check_policy_service` action in this case) doesn't include each individual recipient separately by default. Instead, it sends a single `recipient=` line with an empty value to indicate that there are recipients but doesn't list them individually.
+> There is no way for PolicydRateLimit to register any recipients, neither in the log message nor in the `messages` db table, if a message was sent to multiple email addresses in `To:`. Also, there is no way to register any recipients from `Cc:` and/or `Bcc:` headers.
+>
+> But aside this little drawback, Postfix integration using [`check_policy_service`](https://www.postfix.org/postconf.5.html#check_policy_service) is simply the best and probably the only way to go. Currently, this is the only supported way to configure it!
+
 ### Upgrade
 
 You can run an upgrade with the Ansible Galaxy playbook you have created above, using our [`onlime.policyd_rate_guard`](https://galaxy.ansible.com/onlime/policyd_rate_guard) role: