ISP port blocking practice

Suresh Ramasubramanian ops.lists at
Tue Sep 7 02:22:04 UTC 2010

On Tue, Sep 7, 2010 at 7:29 AM, Randy Bush <randy at> wrote:
> i keep hearing that, but am having a hard time finding supporting data.

Might see the stats from - by AS.  Then compare
the stats on a non port 25 filtered network (they have stats by AS) to
stats on a network that is filtered on port 25

The networks that are filtered on port 25 will of course have any bots
on that network originating spam by other means (social networks,
webmail scripting etc), or other types of nastiness (DDoS etc).  But
you won't find them mailing out direct on port 25.

The bots are very much there - and if the port 25 filtering were to be
taken out, you'd at once see the increase in spam volumes.

Suresh Ramasubramanian (ops.lists at

More information about the NANOG mailing list