[OT] Routing?

[Lennart Sorensen]

On Mon, Jun 27, 2005 at 09:07:25AM -0400, phil wrote:
> (I'm asking this here just because I suspect there are readers with 
> network administration smarts.)
> 
> I have a DSL connection via IStop.  My wife has a Rogers cable 
> connection.  Her e-mail provider is a different ISP altogether.
> 
> When she connects her laptop using cable, POP works and SMTP times out, 
> both using the same host.  Connecting the same laptop using DSL has 
> both services working fine.  If I telnet port 25, it works on DSL and 
> times out on cable.  The host name resolves to the same IP address 
> regardless of which network connection.
> 
> This is a new development, starting over the past few days.
> 
> I'm baffled at the idea that a particular port responds or not, 
> depending on the source address or network.  (I know some providers 
> shut down SMTP from outside their network, but in this case: providing 
> 3rd-party mail is part of their business, there's no error message, and 
> it's strangely selective.)
> 
> Any suggestions of what might be going on (before we call various tech 
> supports and grovel)?

Many ISPs block outgoing smtp connections to prevent mail viruses in
outlook from spreading to easily.  They only allow outgoing smtp to
their mail server so that they can log which IP sent each message and
track them in case of abuse/spam complaints.  Certainly sympatico blocks
smtp, and perhaps rogers has started to do so as well.

Istop does not block anything.  That is probably why it works.

At the same time many dynamic IP ISPs (like rogers) have their user IP
blocks added to blacklists as IPs that should never send mail directly
so many mail servers refuse connections from them.  I find it to be both
a good idea (anything to prevent zombie windows PCs from sending
spam/viruses is a good thing), while also a pain (anything that prevents
advanced users from runing their own mail when their ISP isn't competent
enough is a bad thing).

Lennart Sorensen
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