Comcast blocking IPv6 subnets at SMTP level

Björn JACKE bj at SerNet.DE
Tue Jan 12 09:35:20 CET 2016


did anybody experience issues with Comcast blocking IPv6 addresses at SMTP
level? We used to be able to send mail to comcast servers but since a couple of
days they cancel SMTP connections with a misleading error message:

# telnet 25
Trying 2001:558:fe21:2a::6...
Connected to
Escape character is '^]'.
554 comcast ESMTP server not available
Connection closed by foreign host.

Forward and reverse DNS etc. is all fine at our site. From other IPv6
subnets the SMTP connection isn't teminated.

The KDE project had the same issues last year, see

I already had a one hour call with a Comcast support guy but their support
isn't helpful at all, in the end he didn't know what's wrong and the result of
the call with them was zero. Currently it looks like Comcast needs to be IPv6
blacklisted for SMTP if you want to keep sending mails to them reliably. But
that can't really shouldn't be the solution...


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