There are a few different ways that you can go about this, but I think that the following is the easiest (though not necessarily the least work). The primary advantage is that this gives you plenty of time to migrate the e-mail and you can do the entire company (or domain) at a time or one mailbox at a time at your own pace.
-Create your Google Apps users and groups. Be sure to change the group access level if necessary. I recommend using the Directory Sync tool, but the last time that I used it, which was a while ago, it didn't set the access level on the groups to match the settings in Exchange.
from an authoritative domain to an internal relay domain in Exchange.
-Disable the AddressBookEnabled setting for the internal relay domain. See http://exchangeserverpro.com/exchange-internal-relay-ndr-550/
a send connector for e.co.uk
and set the smarthost to aspmx.l.google.com
-Enable the test domain for Google Apps (e.co.uk.test-google-a.com
-Create mail contacts for all users on e.co.uk
pointing to the corresponding Google Apps test e-mail address for that user.
-Setup forwarding for all users.*At this point, new e-mail is going to both sets of mailboxes*
-Migrate the mailboxes.
While this makes for a pretty smooth transition, there are a few things to keep in mind and you might want to modify the process depending on the timing of the migration.
-Contacts and calendars shouldn't be migrated more than once or else you'll end up with duplicates. I migrate these last as a separate step if doing bulk migrations to avoid a window where changes to contacts were made during the migration.
-If a user moves a message after the initial migration pass, the change won't be reflected even after the next synchronization cycle.
-If you're only going to migrate a few mailboxes at a time, I wouldn't start forwarding messages until you're closer to the migration date for that particular mailbox.
-Instead of forwarding, you could also delete any e-mail addresses for e.co.uk
. However, you could run into issues where Outlook is still using the X.400 address. If you don't have any X.400 addresses, then this is a viable alternative. You could also delete the X.400 addresses and deal with any bounces that might result. For users who already have some kind of forwarding setup that needs to remain, your only choice is to modify the forwarding configuration.
Some additional tips:
-You should audit the forwarding settings just so that you're aware of any complicated forwarding setups before it's time to migrate. You'll want to plan for how it affects the migrated mailbox and forwarding on the Google Apps side.
-Google Apps Sync for Microsoft Outlook is terrible, especially for large mailboxes. I hope that you're planning on migrating to web-only or that user expectations have been properly addressed.
-New mailboxes that need to be created during the migration period just need Google Apps accounts. Don't create the Exchange mailbox and e-mail will just flow from Exchange without the need to create a contact.
-Mailboxes that don't need data migrated can just be disabled (detached) without creating a contact.