nanog mailing list archives

Re: Remote email access


From: Daniel Senie <dts () senie com>
Date: Thu, 30 Jan 2003 22:53:42 -0500


At 10:25 PM 1/30/2003, Eliot Lear wrote:

It's a rare day when I differ with Dave over mail standards, so something's weird.

Dave Crocker wrote:
Some current choices:
Email standards provide for posting of email to the usual port 25 or to
port 773 for the newer "submit" service. (Submit is a clone of SMTP that
operates on a different port and is permitted to evolve independently of
SMTP, in order to tailor posting by originators, differently from
server-to-server email relaying.) There is also a de facto standard for
doing SMTP over SSL on port 465, although this collides with the IANA
assignment of that port to another service.

The submission port, according to IANA is 587. I'm not a fan. I also think experience has shown that it is POSSIBLE to protect port 25 appropriately. It's just a matter of doing it...

See http://www.iana.org/assignments/port-numbers

I am a fan of port 587 being a viable alternative, as it provides a way for our customers who are blocked on port 25 to send their email through our servers (with SMTP AUTH or SMTP-after-POP). If this is going to be the norm from now on, it'd sure be nice if the MUAs had an automated way to set up users to use the SUBMISSION port. It'd also be nice if a certain large vendor fixed their MUA to understand STARTTLS and properly implemented it.

Port 25 blocking, especially on dialups, did for a time cut down on the spam levels. This benefit has largely disappeared as the spammers now use open proxies found all over the 'net.


Standardized SMTP authentication uses the SMTP Auth command or the SASL
service within SMTP. It can also use the de fact "POP hack". All 3 of
these mechanisms are inline -- as part of the posting protocol -- so
that they work over whatever port is being used for posting.
Standardized privacy for SMTP uses SMTP over SSL or it uses SMTP with
SASL.  SASL can be used on any SMTP or Submit port.  SSL can only be
used on port 25 if the SMTP service is not available to other SMTP
servers for relaying (or, really, for last-hop SMTP delivery).

Although Dave is correct about SSL, RFC 3207 discusses the use of TLS for purposes of encryption AND authentication. I use this for my own sendmail. The biggest problem is ensuring that appropriate certificates are installed. Most of the common MUAs I tested have a way to do it, but it's messy (to say the least).

We encourage our users to use STARTTLS, but they're using username/password for the SMTP AUTH (and the POP auth) rather than client certs.


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