I'm resending this with a new subject. The last email just disappeared (no bounce). Maybe having DMARC_ORGANIZATIONAL_DOMAIN_DATA_URL in the subject tripped an all caps in subject test. (Or the moderator is slacking?) By default DMARC_ORGANIZATIONAL_DOMAIN_DATA_URL is set to https://publicsuffix.org/list/public_suffix_list.dat I have mailman set up on an IPv6 only host and publicsuffix.org has no IPv6 address. A near identical configuration is set up on a dual stack host. Any email to the IPv6 only host fails with an entry in logs/error of the form "Unable to retrieve data from https://publicsuffix.org/list/public_suffix_list.dat: <urlopen error [Errno 43] Protocol not supported>" It doesn't look as if publicsuffix.org will be getting an IPv6 address any time soon. The alternative github.com also doesn't have an IPv6 address. In the mean time I would like to disable the use of DMARC_ORGANIZATIONAL_DOMAIN_DATA_URL by setting it empty. To do this it seems that I would need the following change: - --- Mailman/Utils.py.orig 2016-04-09 04:08:56.000000000 -0400 +++ Mailman/Utils.py 2016-05-03 14:37:12.683904000 -0400 @@ -1205,6 +1205,8 @@ Domain which may be the same as the input.""" global s_dict if not s_dict: - get_suffixes(mm_cfg.DMARC_ORGANIZATIONAL_DOMAIN_DATA_URL) + if mm_cfg.DMARC_ORGANIZATIONAL_DOMAIN_DATA_URL: + get_suffixes(mm_cfg.DMARC_ORGANIZATIONAL_DOMAIN_DATA_URL) hits = [] d = domain.lower().split('.') In mm_sfg.py I just set "DMARC_ORGANIZATIONAL_DOMAIN_DATA_URL = ''". This works for .com, .org, .net, etc but not for things like co.uk, etc (which in my case is not an issue). A second question is why does failing to access publicsuffix.org result in a hard fail rather than a soft fail? The change I made just skips over get_suffixes and leaves s_dict empty. It seems that get_suffixes does do a "try and except" which logs and returns, but then the mail gets rejected and the reason is not clear to me by just reading the code. In logs/smtp-failure there is a message of the form "failed with code 554: 5.7.1 <fqdn[ipv6addr]>: Client host rejected: Access denied". Skipping get_suffixes throws no error and leaves s_dict empty. At most an error in get_suffixes should put the email in shunt but that seems to be in error handling upstream. Curtis