Build email flood defenses

src/freedombone-base-email

@@ -2023,4 +2023,16 @@ function email_remove_html {
     fi
 }
 
+function email_flooding_defense {
+    acl_check_rcpt_file=/etc/exim4/conf.d/acl/30_exim4-config_check_rcpt
+    if [ ! -f "$acl_check_rcpt_file" ]; then
+        return
+    fi
+    if grep -q '# Flooding defense' "$acl_check_rcpt_file"; then
+        return
+    fi
+    sed -i "/acl_check_rcpt:/a \\\\n  # Flooding defense\\n  deny\\n    ratelimit = 5 / 15m / per_rcpt / strict / \$sender_address\\n    log_message = Denied (sender address), sending rate of: \$sender_rate by \$sender_address" "$acl_check_rcpt_file"
+    systemctl restart exim4
+}
+
 # NOTE: deliberately no exit 0

src/freedombone-upgrade

@@ -154,6 +154,7 @@ if [ -d "$PROJECT_DIR" ]; then
         populate_keyservers
         disable_email_encryption_at_rest
         email_remove_html
+        email_flooding_defense
         if [ -f /var/log/freedns_@_update.log ]; then
             rm /var/log/freedns_@_update.log
         fi

src/freedombone-utils-setup

@@ -1200,6 +1200,9 @@ function setup_email {
 
     function_check email_remove_html
     email_remove_html
+
+    function_check email_flooding_defense
+    email_flooding_defense
 }
 
 function setup_web {