diff --git a/target/scripts/startup/setup.d/security/rspamd.sh b/target/scripts/startup/setup.d/security/rspamd.sh
index 86786932..db455fc6 100644
--- a/target/scripts/startup/setup.d/security/rspamd.sh
+++ b/target/scripts/startup/setup.d/security/rspamd.sh
@@ -21,6 +21,7 @@ function _setup_rspamd() {
     __rspamd__setup_greylisting
     __rspamd__setup_hfilter_group
     __rspamd__setup_check_authenticated
+    __rspamd__setup_spam_subject
     _rspamd_handle_user_modules_adjustments   # must run last
 
     # only performing checks, no further setup handled from here onwards
@@ -304,6 +305,55 @@ function __rspamd__setup_check_authenticated() {
   fi
 }
 
+# If `SPAM_SUBJECT` is not empty, we create a Sieve script that alters the `Subject`
+# header, in order to prepend a user-defined string.
+function __rspamd__setup_spam_subject() {
+  if [[ -z ${SPAM_SUBJECT} ]]
+  then
+    __rspamd__log 'debug' 'Spam subject is not set - no prefix will be added to spam e-mails'
+  else
+    __rspamd__log 'debug' "Spam subject is set - the prefix '${SPAM_SUBJECT}' will be added to spam e-mails"
+
+    __rspamd__log 'trace' "Enabling '+editheader' Sieve extension"
+    # check whether sieve_global_extensions is disabled (and enabled it if so)
+    sed -i -E 's|#(sieve_global_extensions.*)|\1|' /etc/dovecot/conf.d/90-sieve.conf
+    # then append the extension
+    sedfile -i -E 's|(sieve_global_extensions.*)|\1 +editheader|' /etc/dovecot/conf.d/90-sieve.conf
+
+    # This directory contains Sieve scripts that are executed before user-defined Sieve
+    # scripts run.
+    local DOVECOT_SIEVE_GLOBAL_BEFORE_DIR='/usr/lib/dovecot/sieve-global/before'
+    local DOVECOT_SIEVE_FILE='rspamd_spam_subject'
+    readonly DOVECOT_SIEVE_GLOBAL_BEFORE_DIR DOVECOT_SIEVE_FILE
+
+    mkdir -p "${DOVECOT_SIEVE_GLOBAL_BEFORE_DIR}"
+    # ref: https://superuser.com/a/1502589
+    cat >"${DOVECOT_SIEVE_GLOBAL_BEFORE_DIR}/${DOVECOT_SIEVE_FILE}.sieve" << EOF
+require ["editheader","variables"];
+
+if header :contains "X-Spam" "Yes"
+{
+    # Match the entire subject ...
+    if header :matches "Subject" "*" {
+        # ... to get it in a match group that can then be stored in a variable:
+        set "subject" "\${1}";
+    }
+
+    # We can't "replace" a header, but we can delete (all instances of) it and
+    # re-add (a single instance of) it:
+    deleteheader "Subject";
+
+    # Note that the header is added ":last" (so it won't appear before possible
+    # "Received" headers).
+    addheader :last "Subject" "${SPAM_SUBJECT}\${subject}";
+}
+EOF
+
+    sievec "${DOVECOT_SIEVE_GLOBAL_BEFORE_DIR}/${DOVECOT_SIEVE_FILE}.sieve"
+    chown dovecot:root "${DOVECOT_SIEVE_GLOBAL_BEFORE_DIR}/${DOVECOT_SIEVE_FILE}."{sieve,svbin}
+  fi
+}
+
 # This function performs a simple check: go through DKIM configuration files, acquire
 # all private key file locations and check whether they exist and whether they can be
 # accessed by Rspamd.