[svlug] ISP blocks port 25

Joe Buck Joe.Buck at synopsys.COM
Tue Apr 17 10:13:32 PDT 2007

Skip Evans wrote:
> > I just switched my home ISP from Qwest to Bresnan 
> > on a cable modem, and also discovered they block 
> > port 25 on their home accounts so I am unable to 
> > connect to my SMTP server here in the office on 
> > port 25 to receive mail.

On Tue, Apr 17, 2007 at 09:53:33AM -0700, Florin Andrei wrote:
> Call them and ask them to remove the block.
> If they don't comply, cancel the contract, switch to a less dictatorial 
> ISP, and make sure you let everyone know which ISP to avoid.

Instead, thank them for being proactive in helping to block spam.  The
reason many ISPs block port 25 is that it kills the most common type of
spambot, which infects users' machines and promptly spams the net by
connecting to every SMTP port on the net and spaming it.  So I think it's
a good thing, as a rule, to block that port.  Also, if you have a dynamic
IP address, most sites will black-hole your mail anyway if you do direct
SMTP connections.  If you intend to use a site you own as a relay, you
need to make sure that you don't relay for the whole net, or again you
will be black-holed.

Try working with the ISP to figure out solutions that work for you but
don't harm the net as a whole.  Allowing SMTP to one particular host
is one way to go.  Using authenticated SMTP, which uses a different port,
is another.

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