diff --git a/tests/dummysmtpd.py b/tests/dummysmtpd.py
--- a/tests/dummysmtpd.py
+++ b/tests/dummysmtpd.py
@@ -19,6 +19,8 @@
 )
 
 def log(msg):
+    with open('dsmtpd.log', 'a') as f:
+        f.write(msg)
     sys.stdout.write(msg)
     sys.stdout.flush()
 
diff --git a/tests/test-patchbomb-tls.t b/tests/test-patchbomb-tls.t
--- a/tests/test-patchbomb-tls.t
+++ b/tests/test-patchbomb-tls.t
@@ -124,3 +124,8 @@
   [255]
 
   $ cd ..
+
+This is the log for the entire lifespan of the dummy smtpd.
+
+  $ cat dsmtpd.log
+  listening at localhost:$HGPORT