Reverse dns and SaaS Spam Filters

Question

We have an exchange server in the office that for a year or so has just used the built-in spam filters - we have added the SpamHaus IP addresses so that some basic spam filtering is done. However, this is no longer viable - we just get too much spam.

So we have looked at several SaaS Spam Filtering services, such as MX GuardDog. For it to work we need to change our MX records to point to the MX GuardDog IP addresses. Our setup is currently:

• Host: exchange.mydomain.com
• MX (for mydomain.com) is exchange.mydomain.com

I think this is pretty much as you would expect.

Our static IP address has a reverse DNS entry set to point to exchange.mydomain.com. My concern is that if we have to change our MX records to point to MX GuardDog, will this in itself not pose problems with revere DNS? I suppose, my question is the reverse DNS entry pointing to the host, or the MX record - if that makes sense? Or, put another way - if our MX record changes from exchange.mydomain.com to mx1.guarddog.com are we going to get reverse DNS problems, which in itself could cause certain email servers to block our mails?

Answer 1

MX Records are for incoming mails, reverse DNS is checked for outgoing mails. Sending and receiving doesn't have to be handled through the same server. So if your MX record points to the SAAS Spamfilter, your send path is not affected. You will still send through exchange.mydomain.com and that A-record and PTR record stays the same.

side note: I've had many support cases like "since we changed our MX records to the SAAS Filter, our outgoing mails get rejected" - it was never related to RDNS, but always caused by misconfigured SPF records, so in case you have SPF records set up, make sure they explicitely allow exchange.mydomain.com to send mails. If there currently is a "mx" in your SPF record you probably need to change it for the move.