Sendmail on a LAN

[Gordon Messmer]

On 08/17/2010 09:33 AM, JD wrote:
> Re:  a.b.c.d ==>  valid.host.name
> and valid.host.name ==>  a.b.c.d
> does not seem to apply to the google smtp server I use for Thunderbird.

You did your test entirely backward.  You did a forward lookup first, 
and then checked the PTR of the IP which was returned.  There is no 
requirement for a PTR to match every hostname that resolves to its IP 
address.

Let's finish your test:

$ host smtp.gmail.com
smtp.gmail.com is an alias for gmail-smtp-msa.l.google.com.
gmail-smtp-msa.l.google.com has address 74.125.155.109

The result of this test merely identifies an IP address.  Now, let's 
test to validate that the IP returns a PTR that resolves to the same IP:

$ host 74.125.155.109
109.155.125.74.in-addr.arpa domain name pointer px-in-f109.1e100.net.
$ host px-in-f109.1e100.net.
px-in-f109.1e100.net has address 74.125.155.109

Yep, totally valid.  That IP address has a PTR record, and the hostname 
contained in that PTR resolves back to the same IP address.  This host 
is properly configured.

> So, Thunderbird client does not seem to mind that
> reverse lookup does not match the name smtp.gmail.com

Clients rarely do.  It's the servers to which you're going to try to 
deliver mail that will mind.