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Re: Finding a replacement for my ISP's smtp server


Dňa Mon, 28 Jul 2014 14:56:40 +0100 Brian <ad44@cityscape.co.uk>

> On Mon 28 Jul 2014 at 14:02:29 +0200, Slavko wrote:
> > Dňa Sun, 27 Jul 2014 13:02:18 +0100 Brian <ad44@cityscape.co.uk>
> > napísal:
> > 
> > > He could check with nc.
> > > 
> > >   brian@desktop:~$ nc smtp.gmail.com 25
> > >   220 mx.google.com ESMTP 19sm41008233wjz.3 - gsmtp
> > > 
> > 
> > AFAIK, the port 25 have to used only for (inter-) servers
> > connections, the clients have connect via 587, the port 25 for
> > client connections is for backward compatibility only.
> How does the server tell the difference between talking to another
> server (which is acting as client) and what you call a "client"?

I don't know how server can differ the client and another server, but
from abstract of RFC 6409 (which obsoletes the RFC 4409 which
obsoletes the RFC 2476):

   Message relay is unaffected, and continues to use SMTP over port 25.

   When conforming to this document, message submission uses the
   protocol specified here, normally over port 587.

And latter in the same RFC:

3.  Message Submission

3.1.  Submission Identification

   Port 587 is reserved for email message submission as specified in
   this document.  Messages received on this port are defined to be
   submissions.  The protocol used is ESMTP [SMTP-MTA], with additional
   restrictions or allowances as specified here.

   Although most email clients and servers can be configured to use port
   587 instead of 25, there are cases where this is not possible or
   convenient.  A site MAY choose to use port 25 for message submission
   by designating some hosts to be MSAs and others to be MTAs.

By this i assume, that i posted formerly.

As client i consider the MUA, but e.g. exim has a "client mode" too (i
don't know about other MTAs). I am sorry for simplification.

I want to point, that for end users is intended the 587 port, as
mentioned someone other too, and IMO it is not a good opinion to
suggest to try (check) the port 25, if the 587 is provided. Another
goal is, that there are some ISP, which don't blocks/redirects/proxies
the 587 port yet ;-)



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