We have an SMTP server setup on an internal IP (10.x.x.x) that will send email to our customers. Since the server is inaccessible to DNS lookups from the outside, will this cause issues for sending and/or NDR's? I also told them to set up and SPF record for the domain, but since the server is not public, I doubt it would do any good.


1 Answer 1


If the server has no way of contacting the outside world, then yes this will be a major problem. If you're using NAT, then the server will appear to be connecting from the NAT's external IP.

If you do not have forward and reverse DNS configured properly then many servers will not accept e-mail from your server. If you're server announces itself as mail.example.com then DNS must resolve to the NAT's IP address. Also, the IP's reverse pointer must point to mail.example.com (eg, if your external IP is then must be a PTR to mail.example.com).

SPF records match DNS Host names usually (though you can include IPs, it's not a best practice). SPF records have to be somewhat carefully crafted or else they'll work against you; do take a bit of care in setting one up and it will greatly reduce the number of your outgoing e-mails that end up in people's Junk bins.

  • 1
    Was typing up the same - beat me to it by 30 secs!
    – Robin Gill
    Feb 28, 2012 at 14:04
  • Gotta get something 'good enough' and post it, then go back a fill in the details. =]
    – Chris S
    Feb 28, 2012 at 14:05
  • It can contact the outside world. It's hostname is set up as ics.local. It seems to announce itself as ics.com for whatever reason. That is not even a domain of ours. We are using MailEnable and I don't have direct access to check if that is something they put in because ics.local is not a valid tld. Here is some of the log info EHLO ics.com 250-Postini says hello back 14 67 MAIL FROM:<> 250 Ok 14 8 RCPT TO:<[email protected]> 250 Ok 34 8
    – dbinott
    Feb 28, 2012 at 14:21
  • You should be able to configure what the mail server software announces itself as. I'm not familiar with that software, but the configuration option is usually something like "HELO Name", "Masquerade Name", or "Server Name"
    – Chris S
    Feb 28, 2012 at 14:24
  • So just so I can get back to them with correct information, we will have issues with sending emails to some domains by not having an rDNS set up?
    – dbinott
    Feb 28, 2012 at 14:47

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