diff --git a/README.md b/README.md
index 63a1bd2..83c7711 100644
--- a/README.md
+++ b/README.md
@@ -151,6 +151,12 @@ property. The classification threshold can be adjusted by setting
 required_score 9.0
 ```
 
+In SpamAssassin Milter, the `--reject-spam` option will cause messages flagged
+as spam to be rejected during the SMTP conversation. When rejecting spam in this
+manner, accepted (not rejected) messages will not have the `X-Spam-Flag: YES`
+header; if you need to make a further decision about the message coming through,
+look at the score in the `X-Spam-Level` header instead.
+
 By default, SpamAssassin creates ‘reports’ for messages it recognises as spam.
 These reports replace the message body, that is, the message body is rewritten
 to present a report instead of the original message, and the original message is
@@ -244,7 +250,8 @@ on those.
 
 As an example, in case [Dovecot] does mail delivery using [LMTP], enable the
 Sieve plugin for the LMTP protocol, and then set up a global Sieve script that
-files messages flagged as spam into the ‘Junk’ folder:
+files messages flagged as spam into the ‘Junk’ folder. That is, test for the
+header `X-Spam-Flag: YES`:
 
 ```
 require "fileinto";
@@ -255,6 +262,10 @@ if header "X-Spam-Flag" "YES" {
 }
 ```
 
+Alternatively, use the value of the `X-Spam-Level` header instead. In the
+example above, the test `header :contains "X-Spam-Level" "*****"` would file
+messages with score 5.0 or above into ‘Junk’.
+
 [Sieve]: http://sieve.info
 [Dovecot]: https://dovecot.org
 [LMTP]: https://doc.dovecot.org/configuration_manual/protocols/lmtp_server