Hi NoDoze, pleased to hear you've made the decision to make the jump to Zimbra
First of all, do you need to migrate messages from the current platform onto the new Zimbra server? If so - what sort of system is this and how do your users connect to it - web interface, Outlook, Thunderbird? Are messages stored on the server or on local computers? There's a large article in the wiki about migrating from various platforms -
User Migration.
As for testing; it might be a good idea to set up a temporary domain (use a subdomain of your existing one or use
FreeDNS for example) so it won't affect your current setup. Once you have confirmed mail delivery works fine through your inhouse setup inbound and outbound, then you can plan it "for real".
As for DNS - Zimbra will need to be able to resolve itself to its local IP on the internal network and
not the externally-visible internet IP. Read this
Split DNS wiki article for more information. If you have an internal DNS server though that you can configure the Zimbra server to use, you can probably get away with utilising that instead of configuring Split DNS on the Zimbra server.
As for actual switchover over mail delivery - there are a couple of ways I can see doing this.
Option 1 - Do a straight change of the MX records (when it's ready) to an appropriate A record pointing to your internet IP.
To provide a seamless transition for incoming email, it may be wise to decrease the TTL (if possible) for the MX/A records in advance. This will mean that when you eventually change them, the period between record updates will be less so the changes will 'appear' sooner. Have a read of the
In theory/
In practice section of this -
Wikipedia - Domain Name System. It explains it better than what I can
Option 2 - Have your current email server forward all messages to the new server (again, to an appropriate A record pointing to your internet IP) and optionally change the MX records after the forwarding is confirmed to be working.
The possibility of this will largely depend on what control and access you have to the current email server. I think Exim calls this Smarthosts and I believe Postfix does it using transport maps.
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Don't forget you will need to reconfigure your users' email clients if you use new DNS settings. When I migrated I found it a lot easier to schedule in some downtime well in advance so users are aware of what's going on. This allows for plenty of time to get things sorted and you can bring everything back up for your users when convenient to you and all tested etc. Your mileage may vary depending on what you actually need to do with regards to DNS changes.
Hope that helps and gives you a bit of food for thought.