What happens to email durring account move?

I'm setting up two new servers and decommissioning the single old server I've been using.

I went ahead and setup the new servers w/ cluster dns, and hooked in the old server, so now any account moves get synced across them immediately. And yes, I've changed the IPs from my nsX.... servers to the new server IPs within my zone file and w/ the registrar.

So, when I move an account to a new server, instantly all it's DNS is now pointing to the new box, but of course some servers out there will still have the old IPs until their DNS cache grows stale and they query and get the new IPs. So they'll try to deliver to the old server.

Question - what happens to the email?

I think it's best to not delete the account from the old server for a day or so, that way even web queries that still come to the old server will be served up the pages (of course things like the database won't be synced, so any changes there on the old server will be "lost").

Back to the email, so will the old server accept the email and then realize that it's not the root MX server and forward it on to the new server?

Thanks!

(yes, I did try searching around first, you know how many posts come up for "account move email"? dug through them but didn't get anywhere)