SMTP rate-limits [Was: Re: ingress SMTP]

Michael Thomas mike at
Fri Sep 5 09:46:00 CDT 2008

Paul Ferguson wrote:
> Hash: SHA1
> - -- Simon Waters <simonw at> wrote:
>> If the ISP blocks port 25, then the ISP is taking responsibility for 
> delivering all email sent by a user, and they have to start applying rate 
> limits. Otherwise if they send all email from their users, all they've done
> is take the spam, and mix it in with the legitimate email, making spam 
> filtering harder.
> Okay, I can understand why an ISP might want to apply SMTP
> rate-limits, but to clarify, I'm assuming you meant that ISPs
> (if they do block tcp/25 outbound to anything other than their
> own MTAs) need to watch for excessive SMTP utilization, which might
> indicate a spammer-client (?).
> opposed to arbitrary SMTP rate-limits.
> Yes?
I thought that these bot nets were so massive that it is pretty
easy for them to fly under the radar for quotas, rate limiting, etc.
Not that all bot nets are created equal, and there aren't local hot
spots for whatever reason, but putting on the brakes in a way that
users wouldn't feel pain is simply not going to make any appreciable
difference in the overall mal-rate.



More information about the NANOG mailing list