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 <[email protected]>
wrote:
<snip>
?? 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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