Migrating MX Records

Posted on 2012-08-27
Last Modified: 2012-08-29
I am deploying a new email gateway that will handle TLS and web based email delivery, really all email delivery to/from my company.  Previously I was running exchange only.  I am planning the migration and am trying to decide on the best way to handle DNS(MX) records.  Currently mail is delivered to,, and mail3.domain .com with pref.s of 10, 20, and 30 respectively.  All point via different ISP's to the same mail server internally named  My thought was to name the new mail proxy (and get an SSL cert for said name).  I could create public A & MX records with MX preferences of 15, 25, and 35 for smail, smail2, smail3. Thus until I took exchange out of line for direct email delivery, it would be attempted first.  Once the gateway is up and tested, I could remove the records from the MX list.  I’m just running this by to be sure I’m not missing something obvious and disastrous.  Would this work?
Question by:whoam
    LVL 33

    Accepted Solution

    This would definitely work, however ensure that post each DNS Change you are giving atleast 72hrs for changes to be replicated across to the internet AND ensure that you're ISP guy has made rules that doesn't surprise you.

    LVL 20

    Expert Comment

    The 72 hours given by Exchange_geek should actually rather be taken to be a combination of TTL and SOA entries in your DNS. However, due to the special nature how mail servers treat MX records, you may decommission the old MX records at will - the main effect is rather the deactivation of the mail server.

    I have one feedback question though: Do I understand correctly that you do have three different MX entries because of three redundant uplinks, but there is not actually a backup servre? That is, if your mail server is down, all three MX fail at once?

    Author Closing Comment


    Author Comment


    Good observation, yes only one mail server instance exists so if it fails, all 3 IP's would stop taking smtp.  However, it's virtual on multiple physical hosts and regularly backedup and "snapshotted" on shared storage so could be kick-started quickly.  When we go Exch'13 we'll re-do with multiple front-ends etc. which will be a better design.  Just don't have the budget right now.

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