Clarify IPv6 vs. IPv4 protocol priority for local_address

Closes #152

Author: simonrob

emailproxy.config

@@ -25,10 +25,12 @@ documentation = Local servers are specified as demonstrated below where, for exa
 	(assumed to be False otherwise). With this parameter set, STARTTLS negotiation will be handled by the proxy on your
 	behalf (i.e., do not enable STARTTLS in your client). IMAP STARTTLS and POP STARTTLS are not currently supported.
 
-	- The `local_address` property can be used to set an IP address or hostname for the proxy to listen on (both IPv4
-	and IPv6 are supported). If not specified, this value is set to `localhost`. When running the proxy using python 3.8
-	or later, and in an environment with dual-stack support, the proxy will attempt to listen on both IPv4 and IPv6
-	hosts simultaneously. To request this for the local host, set `local_address = ::`.
+	- The `local_address` property can be used to set an IP address or hostname for the proxy to listen on. Both IPv4
+	and IPv6 are supported. If not specified, this value is set to `localhost`. When using a hostname the proxy will
+	first resolve this to an IP address, preferring IPv6 over IPv4 if both are available. When running in an environment
+	with dual-stack support, the proxy will attempt to listen on both IPv4 and IPv6 hosts simultaneously. To explicitly
+	request this for the local host, set `local_address = ::`. Note also that tools such as `netstat` do not always
+	accurately show dual-stack mode; if in doubt it is worth actually testing both IPv4 and IPv6 connections.
 
 	Advanced server configuration:
 	- In the standard configuration the channel between your email client and the proxy is unencrypted. This is not

emailproxy.py

@@ -6,7 +6,7 @@
 __author__ = 'Simon Robinson'
 __copyright__ = 'Copyright (c) 2022 Simon Robinson'
 __license__ = 'Apache 2.0'
-__version__ = '2023-03-28'  # ISO 8601 (YYYY-MM-DD)
+__version__ = '2023-04-03'  # ISO 8601 (YYYY-MM-DD)
 
 import abc
 import argparse
@@ -1063,7 +1063,7 @@ def handle_error(self):
             # APP_PACKAGE is used when we throw our own SSLError on handshake timeout or socket misconfiguration
             ssl_errors = ['SSLV3_ALERT_BAD_CERTIFICATE', 'PEER_DID_NOT_RETURN_A_CERTIFICATE', 'WRONG_VERSION_NUMBER',
                           'CERTIFICATE_VERIFY_FAILED', 'TLSV1_ALERT_PROTOCOL_VERSION', 'TLSV1_ALERT_UNKNOWN_CA',
-                          APP_PACKAGE]
+                          'UNSUPPORTED_PROTOCOL', APP_PACKAGE]
             if error_type == OSError and value.errno == 0 or issubclass(error_type, ssl.SSLError) and \
                     any(i in value.args[1] for i in ssl_errors):
                 Log.error('Caught connection error in', self.info_string(), ':', error_type, 'with message:', value)