Sendmail Issue

[lists at brimer.org]

On Thu, 1 Sep 2016, Zama Ques wrote:

> Thanks Barry for your quick response.  That very much helps. I need 
> clarification on our configuration scenario where SMARTHOST is not 
> configured in sendmail . So , does that mean that it is querying the MX 
> record in DNS server as configured in /etc/resolv.conf ?  The mail 
> server in MX record is then handling the mail message to deliver it to 
> the destination mailbox.

You're welcome. Yes. That is what that means. Without a SMARTHOST 
configured in sendmail, it will use the DNS servers in resolv.conf and 
query them for the MX records (if available) for the destination domains 
and/or hosts which you are trying to send mail to and deliver it directly 
to them.

Barry

>    On Thursday, 1 September 2016 11:10 AM, Barry Brimer <lists at brimer.org> wrote:
>
>
> <snip>
>> Â  Red Hat Enterprise Linux Server release 6.3 (Santiago)
>
> The current version of RHEL 6 is RHEL 6.8. 6.3 is several years old and
> some very important vulnerabilities have been fixed since then, most
> notably one named Shellshock.
>
>> I am confused how mails are getting delivered since SMARTHOST is not
>> configured . Any pointers ?
>
> A smarthost tells the mail server not to attempt to figure out how to
> deliver mail, i.e. look up MX records and deliver mail, but rather to
> blindly send all outgoing mail to a specific host known as a smarthost
> which will then be responsible for doing the MX lookup and delivering the
> mail.
>
> Think of it this way ... you have one server that is allowed to initiate
> connections out of your company on 25/tcp called outgoing.example.com and
> you have web00.example.com - web19.example.com and app00.example.com -
> app19.example.com. All of these 4 web and app servers need to send mail to
> external customers, but they can't do so directly due to firewall
> restrictions, so you set them all to use a smarthost (known as a relayhost
> in Postfix) of outgoing.example.com. Then when any of those 40 servers
> needs to send an email to a yahoo.com (or any other external) address, it
> doesn't look up the address, it blindly sends it to outgoing.example.com
> at which point outgoing.example.com does an MX lookup for the mail servers
> that handle yahoo.com email and sends the message to the Yahoo mail server
> with the lowest priority.
>
> Does that make sense?
>
> Hope this helps.
>
> Barry
>
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