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

On 7/28/2014 1:16 PM, Brian wrote:
> On Mon 28 Jul 2014 at 10:34:03 -0400, Jerry Stuckle wrote:
>> On 7/28/2014 9:56 AM, Brian wrote:
>>> 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.
> Would I be correct in thinking MTAs only talk to each other over port
> 25?

That is correct.

> Would I also be right that using port 587 mandates authentication
> whereas with port 25 it is optional?

In a correctly operating MTA, yes.

>> 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.
> Port 25 then becomes used only for incoming messages to be sent to
> domains the server is responsible for? If so, that doesn't appear any
> different from the present situation. For relaying a login is perfectly
> understanable, but it can be done on port 25 too. What makes port 587
> necessary?

This became necessary due to the number of trojans used by spammers to
compromise unsuspecting users.  As a result, many ISP's now block any
outgoing requests to port 25.  In my area, both Verizon and Comcast
(Xfinity) do, for instance.

> All my mail from home is sent directly using exim which, as far as I can
> make out will only send on port 25. Leaving aside what you say below (my
> ISP does not block outgoing port 25 traffic) I should not be affected?

Exim can use other ports also.  It's all in the configuration. (but
sorry, I do not have enough expertise to tell you exactly how to do it).

>> 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.
> So, in world where every ISP blocks outgoing port 25 connections the
> delivering of one's own mail becomes impossible. The flow of spam and
> malware across the net will continue to increase though, I suppose.

No, it just means you need to connect to a mail server via port 587,
then have it send the email.

If spammers can't use compromised machines, it severely limits the
number of servers they can use.  And since an IP can be blacklisted if
too much spam is sent through it, responsible hosting companies and ISPs
(i.e. those who don't wish to be blacklisted) will limit the number of
messages which can be sent per time unit and/or terminate accounts for
sending spam.  A user on a compromised machine, though, wouldn't know
their system was blacklisted and probably wouldn't care.

Just another way to help protect unsuspecting users from themselves.


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