Blocking MX query

William Herrin bill at
Tue Sep 4 18:55:32 UTC 2012

On Tue, Sep 4, 2012 at 12:59 PM, Michael Thomas <mike at> wrote:
> On 09/04/2012 05:05 AM, William Herrin wrote:
>> There are no "good" subscribers trying to send email direct to a
>> remote port 25 from behind a NAT. The "good" subscribers are either
>> using your local smart host or they're using TCP port 587 on their
>> remote mail server. You may safely block outbound TCP with a
>> destination of port 25 from behind your NAT without harming reasonable
>> use of your network.
> Would that were true going forward. Consider a world where your
> home is chock full of purpose built devices, most likely with an
> embedded web browser for configuration where you have a
> username/password for each. In the web world this works because
> there is a hidden assumption that you can use email for user/password
> reset/recovery and that it works well.

Hi Mike,

A. What device do you offer as an example of this? I haven't stumbled
across one yet. Web sites yes. Physical home devices, no.

What I *have* seen is devices that call out to a web server, you make
an account on the remote web server to configure them and then all the
normal rules about accounts on remote web servers apply.

B. Bad hidden assumption. Expect it to fail as more than a few cable
and DSL providers are blocking random port 25 outbound. Besides, some
folks change email accounts like they change underwear. Relying on
that email address still working a year from now is not smart.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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