Outgoing SMTP Servers

Mark Andrews marka at isc.org
Thu Oct 27 01:01:16 UTC 2011

In message <4EA8A021.9000805 at blakjak.net>, Mark Foster writes:
> On 27/10/11 11:11, Mark Andrews wrote:
> > In message <op.v3y8xvo6tfhldh at rbeam.xactional.com>, "Ricky Beam" writes:
> >> On Tue, 25 Oct 2011 15:52:46 -0400, Alex Harrowell <a.harrowell at gmail.com>
> >> wrote:>
> >>> Why do they do that?
> >> You'd have to ask them.  Or more accurately, you'd need to ask their  
> >> system integrator -- I've never seen an "in house" network run like that. 
> >> (and for the record, they were charging for that shitty network access.)
> >>
> >> Bottom line: Blocking port 25 (smtp) is undesirable, but necessary for a  
> >> modern consumer internet. (Translation: It f'ing works.) This is the ISP  
> >> saying, "You aren't a mail *server*."  
> > MTA == Mail Transfer Agent.  You don't have to be a *server* to be
> > a MTA.  Blocking SMTP also prevents your customers running encrypted
> > mail sessions to prevent nosy ISP's and others looking at what they
> > are sending.  With DNSSEC now being deployed and DANE being
> > standardised, running a SMTP session with STARTTLS is being a
> > reality.
> Perhaps i'm asking the obvious, but why is "Blocking SMTP" going to
> prevent customers running encrypted mail sessions?
> If SMTP = 25/TCP and encrypted mail sessions run on another port,
> they're not blocked?

It's encrypted email direct to MX.  With that I can know that the email
has been delivered to their mail exchangers.

> > Now most people don't care about this but you shouldn't have to get
> > a business grade service just to have secure email sessions and if
> > you want to run a SMTP server to do that you are not changing the
> > amount of traffic going over the connection so why the hell should
> > a ISP care.  IMAP, POP, SMTP all have about the same overhead for
> > inbound email.
> The majority of consumers will use the SMTP service their ISP provides
> and not look twice.
> Surely anyone wanting to use something different will either
> a) run their own mail server, requiring a static IP address and simply
> requiring the ISP to flick a switch which says 'ok, you're not blocked
> for 25/TCP anymore'

And lots of ISP's don't offer those knobs in the misguided view that
individuals don't need them.

> b) use an alternative SMTP server on port != 25/TCP with their own
> authentication layer and responsibility thereof.

Which is just a non-starter.
> Sometimes I feel like contributors to NANOG see themselves as typical
> users.  IT Engineers are anything but typical when you compare to
> mom-and-pop-interwebs-user, and it's those very users who're likely to
> wind up with malware that'll be firing to random external SMTP servers
> via 25/TCP, delivering spam which is quite effectively blocked by a
> 25/TCP block.

As I said, most don't need it but for the few that do it should be
available and shouldn't require one to get a business service.  It's
like ISP's that won't deliver business grade services to homes.
Just because someone doesn't meet you pre-conceptions of their need
it doesn't mean that what they need is unreasonable.

> I've seen recently SMTP-AUTH sessions exploited (user/pass credentials
> borrowed) for spam purposes, but at least this is an order of magnitude
> more difficult for the spammer, and more easily tracked by the ISP than
> having to do IP/Time based records checks.
> >> MUA's (mail clients) should only be  
> >> connecting to specified MSA's or MTA's (mail *servers*).  They should  
> >> never be connecting to random MTA's (presumably for direct delivery, which
> >> is the job of an MTA not MUA.) The only people who can effectively police 
> >> this is the ISP.
> > Total utter BS.
> Why? It's a reasonable position; end users in the generic sense are
> sending to whatever their client has set up for SMTP, fire-and-forget.  
> Again, I feel like folks are taking their relatively complicated
> use-cases and treating them as the norm.

It's ths whole attitude that end users are incapable on doing thing
correctly.   Most user are prefectly fine with having their mail
go through a ISP's servers but there are exceptions and when people
start say "only a ISP can do this" or "only business need this" by
BS detector goes off because individuals do need to do the same
sorts of things.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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