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

On 7/28/2014 9:56 AM, Brian wrote:
> 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"?

It doesn't, but operation is quite different.  MTA's typically require
no login on port 25, but only allow messages to be sent to domains it
serves (otherwise it quickly becomes a spam server).  Port 587 requires
a login, but allows messages to be relayed to any domain.

Now, for historic reasons, some MTA's still allow login on port 25
(either directly or some indirect method like accessing a POP or IMAP
account before sending).  But these are becoming fewer and fewer.

BTW, many ISP's have blocked outgoing port 25 connections (especially on
residential accounts) because there are a lot of trojans out there which
will install a minimal MTA on a user's machine, unbeknownst to the user.
 This allows spammers to use the compromised machine to be a spam
source, hiding the real source of the spam.


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