Although an ISP mailserver allows its client customers to
connect on ports other than port 25, the mailserver-to-mailserver
(non-client) traffic always requires use of port 25. For any email
to be delivered, a minimum of two mailservers are involved,
hence the malware mailserver on a PC can't talk directly
to the destination mailserver unless it uses port 25,
which will be blocked by responsible ISPs.
Gary VanderMolen
----- Original Message -----
>> AFAIK, most ISPs no longer permit dynamic IP clients to run mail servers.
>
> I think you missed my point, specifically that a large percentage of the
> problem comes from zombie bots on dynamic ip machines. Since many isp's now
> block outbound port 25 on consumer connections, many of the malware programs
> will use random ports to get around the isp smtp blockage.
>
> Also if one wants to run a mail server on a dynamic connection, there is
> generally nothing stopping you, AUP and port 25 blocking notwithstanding.
>
> Get a free dyndns.org account, set the mailserver to run on a different port
> and you are pretty much good to go.
>
> There are very few isp's who actually use deep packet inspection that would
> catch the smtp traffic running on other nonstandard ports.
>
> Now with that said, there are other standard smtp ports that very few isp's
> block, the message submission port on 587 being one. Plus there are ssl/tls
> ports like 465 etc.
--
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