Sendmail on a LAN

Gordon Messmer yinyang at
Tue Aug 17 15:47:54 UTC 2010

On 08/16/2010 10:46 AM, JD wrote:
> Clearly, a full setup of DNS server for your domain
> must be set up, per this wiki, along with mx records ...etc.
> Does this prevent one from settiing up and using sendmail
> on a LAN to send and receive email to/from the outside world?

Not by itself, but I don't like the advice in that tutorial.  It 
suggests a configuration with a catch-all address.  Long-term, you'll 
find that you have to turn this off or else your catch-all will receive 
an enormous amount of spam sent by spammers who used a dictionary attack 
and found that every address they test is valid on your system.  Without 
a catch-all, there's no purpose in using the virtual user feature at 
all, so the howto is somewhat more complicated than it needs to be.

Beyond that, it does not address several practical concerns with setting 
up a mail server.  First, you'll need a static address and a proper PTR 
for it.  You won't be able to set up a PTR without a static address, and 
if the reverse lookup for your IP address isn't valid, many systems will 
refuse your mail.  The reverse lookup (PTR) for your address must be a 
hostname that resolves to your IP. -> PTR -> A

You should also look up your IP address on a blacklist watcher:

If you're listed in one of the dynamic IP blacklists, you want to set up 
a smart host through which you'll relay mail.  Many sites will otherwise 
reject your messages.  Likewise, if your ISP prevents you from making 
outbound connections to port 25, you'll need to use a smart host, 
because you couldn't send mail any other way.

> I understand that some things need to  be set up so that sendmail
> sends headers that use a routable IP address as the source of
> the message. Is it possible to make sendmail use my router's
> public IP address in the message headers? How?

No, that's not quite right.  Sendmail should be configured to use a 
valid hostname for HELO, but the IP address will be recorded in a header 
which is set by the receiver of the message.  You can't do anything to 
change that.

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