For some reason, people often underestimate the value of Split Delivery feature in Postini. While it's a little more complicated to setup, it offers a really strong value for distributed organizations and Google Apps migrations.
In this post I will cover how Split Delivery helps with Google Apps migrations and makes them more flexible and easier to manage.
First of all, what is Split Delivery? This is basic Postini's feature which allows you to split delivery of inbound mail flow to several email configs (i.e. server mail servers). The mail server can be an Microsoft Exchange server or Google Apps.
Before we start the migration to Google Apps, your mail flow in being handled by some sort of mail security solution, often referenced as mail-relay. In rare cases, it's being routed directly to your mail server. Given the fact that most of the migrations are phased (i.e. not all of your users are being migrated overnight), you'll need a flexibly way to route inbound mail.
By flexible way, I mean that inbound email flow for users who are already migrated to Google Apps will be delivered directly to Google Apps and for the rest of your users it will be delivered to your old mail server.
To start, you will need a Stand-Alone version of Postini. The Integrated version doesn't support multiple email configs, thus is inappropriate for Split Delivery. Your Google Apps for Business already includes a license for Google Message Security and you'll need to contact Google (if you purchased it directly) or to your reseller to ask to reestablish your Postini account as Stand Alone. This process can take few days, depending if your domain was registered with Postini prior to the change or not. See more details on two editions of Postini here.
When your Postini account is ready, you'll need to create two separate email configs. Usually, you'll already have one email config, named [Your Account Name] Email Config 1. You can rename it so something else to better reflect it's purpose under "General Settings" tab in your Postini Console.
The idea is to create two email configs, one for your existing mail server and another one for Google Apps.
Configuration of Email Config 1 (Inbound Servers -> Delivery Manager -> Edit) should contain your existing mail servers, i.e.
The 2nd Email Config (again, Inbound Servers -> Delivery Manager -> Edit) should point to Google Apps:
Before you begin your migration to Google Apps, you should place your users under Email Config 1 and as you go thru your migration, all you'll need to do is to move your migrated users between both Email Configs.
Personally, I like to create a group in Active Directory, - something like YetMigratedUsers and sync users using GADS based on group membership. This way, you'll assign all users to this group and remove their membership as soon as you migrate them to Google Apps. The GADS will do the account move between Email Configs automatically.