[SGVLUG] email: Can I send it out directly?

[Jeff Carlson]

David Lawyer wrote:
> I think my email is working OK now, but I'm just curious about this.
> Right now I use LA Freenet (LAFN) as my ISP and use their server to
> send out my email.  This server is where my incoming email goes to
> since I'm offline most of the time.  So it's clear that I have to use
> POP or IMAP and get my email from them.

Well, that only makes sense, since POP and IMAP are mail retrieval 
protocols.

> But what about my outgoing email?  When I dialup to my ISP use the
> internet, I'm connected to the internet.  Exim (or sendmail, etc.)
> could just send out my outgoing email to the Internet, bypassing the
> mail server at LAFN.  I haven't set it up this way, but if I did, will
> it work?

I think this has been discussed already but let me put it in concrete 
terms.  Having been an admin for nearly a hundred email domains, I would 
seriously block any DHCP or PPP originating address I found.  I would 
also block any IP without reverse DNS (a PTR record) whenever I could. 
The odds of you being an innocent victim trying to use your IP for 
sending legitimate email when so many IPs identical to yours are sending 
spam are too small for me to make an exception for you.

The solution for someone in your position is, as has been pointed out, 
to let your ISP handle delivery for you.  When you are running your own 
MTA, this configuration is called a Smart Host.  The "smart host" is 
your ISP's server, which relays for you.  Your Exim will queue up your 
messages, when you log in, you can have it flush its queue, it will 
contact your ISP's relay server, and that's the end of your involvement.

> If I did this, my exim would first use DNS to find the mail servers
> that I need to connect to and send packets to port 25 (ordinary email,
> not TLS) on various mail servers.  If they accept my email, then I've
> succeeded.  But will this work OK?  Will they accept mail from an IP
> number that is roving (assigned to my isp)?  What if my email is not
> accepted and returned to my temporary ip# just as I hang up my ppp
> connection.  Then who gets the return packets?  Not me since I'm
> offline.

It has already been pointed out, no message is sent if the server did 
not accept your message in its entirety.

However, you are sending a message as dave at lafn.org, right?  Well, a 
bounce will go to the MX for lafn.org.  It's that simple.

> As you can see, I don't know much about email servers although I
> supposedly run one.  Per my logs, attempts have been made to relay
> email from foreign countries thru my PC but it's been denied.

You should configure your mail server so that connections are only 
accepted on the loopback interface.  This is pretty easy with sendmail, 
I have no idea about other MTAs.  But if you use fetchmail, which hands 
off messages to Exim on the local machine, no external connection is 
necessary.  And then use Smart Hosting to send to the ISP's mail server, 
and you're never accepting Internet-originating connections.  It really 
can be that simple.

> Don't spend any effort checking out my questions since I'm just
> curious and don't really have any need to send email directly.  But I
> was just wondering ...

The effort has already been spent as an admin responsible for email for 
the last five years.  I'm just catching up on email after being out of 
town for the weekend.