Sendmail Issue

[Barry Brimer]

<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