diff --git a/public/ispmail.sh b/public/ispmail.sh
index a8f0acc..6cf5eef 100755
--- a/public/ispmail.sh
+++ b/public/ispmail.sh
@@ -17,7 +17,8 @@
 # Add quota setup
 #
 # Version 0.4 / 2026-08-01
-# Add smtpd_sender_login_maps to allow sending as an alias address
+# Add smtpd_sender_login_maps to allow sending as an alias address.
+# Add rspamd's allow_username_mismatch to enable DKIM signing when sending as an alias.
 #
 
 usage() {
@@ -864,6 +865,7 @@ dkim_config() {
   cat > /etc/rspamd/local.d/dkim_signing.conf << 'EOF'
 path = "/var/lib/rspamd/dkim/$domain.$selector.key";
 selector_map = "/etc/rspamd/dkim_selectors.map";
+allow_username_mismatch = true;
 EOF
 
   # TODO: create a key for $FQDN and test it