Looking at options for migrating from our existing POP3 Mercury mail system to Zimbra, specifically how to transition MX records smoothly. Mercury mail is currently set to a priority of 10 in public dns, while Zimbra is set to 20. The same domain exists on both systems as well as the same users. Currently, I am prohibiting mail flow from going to Zimbra by keeping SMTP port 25 closed on for Zimbra's IP on our Juniper firewall facing the internet. We are a small company with about 25 users.

I have always been under the impression that public dns propogation can take some time, up to 72 hours which can make MX changes tricky when it comes to migrating mail systems. I have reviewed the Split Domain article on how to handle this situation with forwarding and it seems like a bit of overkill for a small environment like ours.

My question is since I have the MX record already in place, would it be easier to simply block SMTP port 25 on the firewall to the old system when I wish to migrate? I can't shut it down completely, since other services are hosted there. That way mail would bypass the primary MX and flow to the secondary MX, Zimbra, and all I would be left to do is change user client settings to point to Zimbra. Of course I would make the changes to reflect Zimbra as the primary MX, but would not be dependent on DNS propogation for desired mail flow. What am I missing here?