Comcast blocking IPv6 subnets at SMTP level

Björn JACKE bj at SerNet.DE
Tue Jan 12 12:26:55 CET 2016

On 2016-01-12 at 10:04 +0000 Tim Chown sent off:
> We have fallen foul of senderbase's reputation, more than once, which oddly keeps separate reputation for IPv4 and IPv6 transport for the same sending system.  Are you blocked for IPv4 for the same subnet?

it's for 2a01:4f8:192:486::/64 and of actually. IPv4 is fine but as
just the SMTP layer disconnects with a permanent error there is no fallback to
IPv4 at all.

> does show a 220% rise in email volume the past 24 hours, but your rep seems fine, at least for IPv4 as shown there. Also seems fine at spamhaus, though you don’t say which specific sender you have issues with to look that up - might be interesting to see what it says, whether or not Comcast use that particular system.

our reputation is fine according to:

I suspect like with the KDE mailing list server, Comcast just applies stupid
smtp policies to IPv6 addresses in order to "block spam", I haven't seen or
read about this for IPv4 with Comcast before.


More information about the ipv6-ops mailing list