diff --git a/contrib/testing/smtp-dane-mta-sts-check.py b/contrib/testing/smtp-dane-mta-sts-check.py
index f6995cc..0221cf3 100755
--- a/contrib/testing/smtp-dane-mta-sts-check.py
+++ b/contrib/testing/smtp-dane-mta-sts-check.py
@@ -34,7 +34,8 @@ TOKEN = f"smtp27-{os.getpid()}-{int(time.time())}"
 ZONE = "smtp27.test"
 DNS_RESOLVER = "127.0.0.53"
 DNS_AUTHORITY = "127.0.0.54"
-DNS_AUTHORITY_PORT = 5353
+# Avoid mDNS' wildcard UDP/5353 listener, which may be active on desktops.
+DNS_AUTHORITY_PORT = 15353
 HTTPS_ADDRESS = "127.0.0.9"
 TIMEOUT = float(os.environ.get("BONGO_TEST_TIMEOUT", "75"))
 STARTUP_SETTLE = float(os.environ.get("BONGO_TEST_STARTUP_SETTLE", "3"))