North American Network Operators Group

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Re: SMTP Port Blocking: Success or Failure?

  • From: David Burns
  • Date: Mon Feb 28 01:13:11 2005
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On Sat, 26 Feb 2005 19:04:26 -0600, Claydon, Tom
<[email protected]> wrote:
> We are considering filtering outbound SMTP traffic from our ISP
> customers, except from our own mail servers, to help reduce the amount
> of spam originating from our network. How successful/unsucessful has
> implementing outbound SMTP filtering done in stopping or slowing down
> spam from your network?

At Portland State University, we saw a huge reduction in outgoing spam
when we blocked port 25, even with liberal exceptions for everyone who
said they wanted one.  According to SenderBase, the mail volume from
our /16 dropped by half (5.3 to 5.0) .  I don't think there was any
significant drop in legitimate email.

There have been a few problems with ISPs that don't accept submission
or SMTPS, but the support burden for that is way less than responding
to all those spam complaints, and way less than the burden if our
campus had been widely blacklisted (and I think we must have been
pretty close.)

---David Burns