You may want to read the following post that I am linking to, it provides some gotchas and hints on how to make your migration successful.
But there’s no substitute for experience, and although no two migrations are ever the same, here’s my top list of my ‘surprises’ from an email migration running into the tens of thousands of mailboxes. You may never encounter them, nor may I again, but maybe, just maybe it’ll save you a 1AM conference call…
1) You really, really need to understand your user profile
Don’t rely on the Microsoft defaults provided with the Calculator. You have, I assume, an Exchange environment already, and that you can go out there and measure. Once you have these stats, you might find that the idea of hosting 20,000 mailboxes on the old P3 laptop you’ve found in the corner of the office isn’t going to fly. Or more likely, you will find that your initially generous assumptions about deleted item retention and mailbox recovery might need to be trimmed a bit, and log file disks and required IOPS bumped a little. Or a lot.
2) Firewalls need love, too.
Traditionally, a firewall would be put between the bad guys on the internet, and the internal network, and perhaps some partner organisations. However, in a diverse network arrangement, it’s quite common that there might be a firewall between your internal client machines and your CAS’. Your firewall guys will be wise to the fact that a ‘traditional’ outlook client connection uses MAPI based on RPC, in which we’ll look to use TCP/135 and high ports. So, bang the protocols and destination IP addresses in the firewall, and away we go?!
So based on the information you just read, I think you want to head on over to, “What your mother never told you about Exchange 2010 Migrations”