Try `Exception` exceptioin.

And a traceback?

docs/courier-to-mailman3.md

@@ -31,4 +31,5 @@ This is our current version trying to communicate between courier and mailman3.
 * OK, parsing port number, and setting default.
 * Logging error (I hope), when lmtp fails to be created.
   * Maybe this is better?  
-  * Catch exception, log based on that?
+  * Catch exception, log based on that?
+  * Hm, go with bare "Exception"...  The catch wasn't what was failing.

source/courier-to-mailman3.py

@@ -65,7 +65,8 @@ try:
         # 5xx error
         else:
             raise
-except smtplib.SMTPException as e:
+except Exception as e:
     logger.error('lmtp error')
+    logger.error(traceback.format_exc())
     logger.error('lmtp failed errorno: %s, message: %s', e.errno, e.strerror)
     exit(100)