How to move Exchange 2003 mailbox or store from 2003 to 2007 on separate networks

1. Yes you can continue to use your 03 box as a mail server. You can also have it as your "SMTP Gateway/Transport" still if it is where all your email comes into from the internet.

2. OWA will be tricky since you can only have it pointing to a single server. So existing mailboxes on the 2003 box won't be able to use OWA until they are migrated (unless you go through a big ordeal that I don't want to describe here).

3. You will need a certificate for Outlook Anywhere. Regardless, you'll need to setup autodiscover.domain.com DNS records externally for Outlook Anywhere to work properly automagically.

Things to note from our migration:

-Blackberry stuff can be tricky, move the BESAdmin account first. -Activesync relies on the Client Access stuff, so either move them all first, or all last. This will affect OWA though since the same rules apply. -if you have a small enough company (under 500 people) I would strongly recommend moving around 25-50 on a weekend, let them test throughout the next 2 weeks, then move EVERYONE else in a single weekend. -you shouldn't have to mess with their outlook profiles -Migrate resource mailboxes over and then convert them to true "resource mailboxes" using Powershell and OWA

The things that I remember vividly is that OWA/Activesync definitely broke if you moved it over to the 07 box first. So we ended up moving all of the OWA/Activesync users at the same time and then moving the firewall rules to redirect to the 07 box.

EDIT: I'm referring to what happens to OWA if you only have a single Exchange 07 server and don't bother to setup 2 different URLs and redirects for OWA clients. See the technet article I refer to in the comments below.

You can use Move-Mailbox across Forests, you need to make use of the -SourceForestCredential parameter (this all has to be done in the Exchange Management Shell, not console). This article explains in detail how to do it.

The account that is doing the Migration needs to have Exchange Recipient Administrators role or above in both source and target domain, but you do not necessarily need a trust setup. This article contains further information and some useful tips on how things need to be setup for it to work.

However, because of your need to merge the mailboxes and keep them on your old server, this may not be the way to go though, particularly as the user accounts have already been created on your new server. You might be better of using the export-mailbox command to export all your mailboxes to PST files, move them to the new server, and then import them using the Import-mailbox command.

I'm not sure how many mailboxes, or how much data you have, so this may or may not be practical for you, but it will achieve both your aims of keeping the old mailboxes, and merging the data.