It probably depends on your setup.
Is zimbra behind a firewall - so operating on an internal network - where the default gateway address is going to remain the same ? If this is the case, your zimbra setup is not going to change (unless you use an outgoing mail relay)
If your zimbra server is exposed directly to the outside world - that might change things a bit.
We recently changed ISP in the first scenario listed. We have an external IP for mail on our firewall/router that is static linked to our internal zimbra server. So, in our case, everything on our internal network was remaining the same - just the external IP was changing.
It will also depend on who is your domain registrar - as they will be responsible for changing either the IP addresses, or the delegated hosts for your network.
What we did ...
Get the new addresses from your new provider. Have them create reverse DNS lookups for your hosts using their new addresses (otherwise some outgoing emails will defer after the change) Note - Your Domain Registrar is responsible for resolving hostnames to IP addresses (or delegating that responsibility to your new ISP). Your ISP is responsible for the reverse lookups. If you have separate Domain Registrar/ISPs then the new ISP should also be able to create the forward lookups in their DNS in advance of any switchover. We didn't know about the reverse lookup step until I noticed some deferred emails and tried to figure out what the issue was.
At least 24 hours in advance of the change, contact your existing ISP, and have them reduce the TTL on your DNS records down to 300 seconds (this minimum as far as I am aware). Depending on the TTL they had previously set on your records, this can take anything up to 24 hours to propagate out across the internet. This will mean that for any incoming email, when the sending server queries your hostname, the worst case scenario is that it will get the old IP address returned from a downstream DNS server for 5 minutes. That IP address should be uncontactable, so the mail will defer. Assuming everything goes smoothly, on the first retry, it will resolve to the new IP address and the email will come through.
Also contact your domain registrar (In our case the old ISP was also the domain registrar) If your registrar is separate from your ISP - the registrar will have to update their delegation records for resolving your hostnames from one ISP to another. Im not sure if this has any time constraints like the DNS does - so ask them if it applies to you.
Before the formal switch - schedule a brief window where you can reconfigure your router's external IP addresses, and test connectivity from the outside-in and the inside-out for all essential services. You will have to use IP addresses (your new ones) to test from the outside-in. Check you can log onto webmail, and use an email client like Thunderbird to test IMAP, SMTPS, etc. Also check any other services outside of email you need to make sure work. If you are using delegated DNS resolution, connect to your new ISP's dns server, and perform lookups on your hostnames to ensure the correct IP addresses are being returned.
After testing, reconfigure your router IP(s) back to the old one(s) and verify everything is good again. If any of the new connectivity failed, address the issues, and re-run the tests another time.
Once you are ready to make the switch, you will need your Domain Registrar on a conference call, along with your new ISP (in case anything comes up after the switch). The domain registrar must be ready to push DNS record changes out in real-time for you - so prep them for that before the call.
First, make the changes to your routers external IP(s), and once again test the connections - and when satisfied, request your Registrar to change the IP addresses for your hosts in their DNS, and push out the new records. If they are delegating responsibility for the DNS resolution to your ISP's nameservers, they will need to push out the change of responsibility in the same way.
Anywhere between 0 seconds and 5 minutes after the change is pushed out, the outside world should start seeing your new addresses returned for their queries.
I hope this helps. And, good luck with the migration.  | 
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