SMTP server or "forwarding"?

[Paul Howarth]

Jonathan Berry wrote:
> On 8/27/05, Craig White <craigwhite at azapple.com> wrote:
> 
>>On Sat, 2005-08-27 at 00:11 -0500, Jonathan Berry wrote:
>>
>>>Hey everyone,
>>>
>>>Okay, a lot of ISPs now block port 25 out to anything other than their
>>>SMTP server.  In some situations, it would be nice to circumvent this
>>>to get to another SMTP server if one is not available.  So what I had
>>>though is to setup my FC4 linux box to listen for SMTP traffic on a
>>>non-standard port.  Actually, I could just have my hardware router
>>>forward whatever port to 25 on the computer, so the non-standard port
>>>part should be easy.  It would be nice to have a workable solution
>>>with as little as possible.  Does anyone know of some way that I could
>>>maybe take any traffic to my server on my chosen high port and forward
>>>it along to my ISP's SMTP server on port 25?  It sounds possible, but
>>>sketchy enough to where it might not be.  Any ideas?  I figure I could
>>>always just setup my own SMTP server and that should work.  But I
>>>would need to make sure I did that right as I do not want to aid in
>>>the spread of spam and/or viruses.  Since it would be on a strange
>>>port, it shouldn't be as big a problem, if at all.  Any ideas on that
>>>point?  So, what do you think of my idea and options?  If I were to go
>>>the route of setting up my own SMTP server (perhaps even so far as a
>>>whole email server) any tips as to where to start looking for info on
>>>doing this right?
>>
>>----
>>There's often no reason to do what you are speaking of.
>>
>>You can set Postfix or Sendmail to use your ISP's smtp server as a
>>'smart host' so outbound email will be delivered. Then other computers
>>can use the system running Postfix or Sendmail delivering via the smart
>>host can send mail. The only issue is computers that aren't on your
>>local lan can't use your mail server to relay mail - which stands to
>>reason.
>>
>>If for some reason you really believe you need to do what you are
>>asking, the following is offered in sendmail.mc (and I presume something
>>similar is configurable for postfix)
>>
> 
> [snip]
> 
>>dnl DAEMON_OPTIONS(`Port=smtps, Name=TLSMTA, M=s')dnl
>>
>>and at this point - the ISP's blocking ports 25, 80, 137-139, 445 are
>>not blocking 587
>>
>>Craig
> 
> 
> Okay, well I guess I should stop trying to be general and explain the
> situation.  My sister is away at college right now and she has to
> connect to the internet through the university's network.  Of course,
> they block port 25 out to our (home) ISP's SMTP from their network. 
> The university supplies students with email, but this has to be
> accessed via a web interface.  They provide IMAP connectivity for
> professors, but not students (who knows the reasoning for some
> university policies?).  So to my knowledge, there is no SMTP server
> that she could use to send email with a normal client (she uses
> Windows by the way).  So, what I want is to setup something to where
> should could send email to my Linux server, which would then send it
> out to my ISP's SMTP server to go wherever it needs to go.  But since
> port 25 out is blocked, my server needs to listen on a different port.
>  I was just going to pick some random, high number like 4539 for
> instance.  So this is the desired path:
> Computer on non-standard port
> University LAN
> Internet
> My Public IP
> My Local LAN
> Linux Server
> ISP's SMTP
> Continue just like I was connecting directly to the SMTP server
> 
> Does that make sense now why I want this?  Is something like this possible?

Yes, and the "standard" approach for this is to provide an SMTP 
submission service on your Linux server (port 587). Your sister 
authenticates to your server using SMTP AUTH, submits her mail just like 
she would with a regular SMTP server and then your server gets on with 
the job of delivering it.

If you're using sendmail, search for "MSA" in /etc/mail/sendmail.mc

You'll need to set up SMTP AUTH for yourself, but google is your friend 
there.

Paul.