Using Apps Mail and Email Software
As the Google Apps Administrator you can configure your domain for POP/IMAP email access. This will allow your clients to bypass Google Apps Mail and use Outlook, Apple Mail, Thunderbird, or other POP-compliant software. Pretty cool when you think about it. The Apps Mail exam will test your knowledge of POP and IMAP settings, the differences between the two, and how to configure Apps Mail for these protocols.
You can also take all your messages from Microsoft Exchange, for example, and import it to Apps Mail. Google Apps Sync for Microsoft Outlook will let you migrate your mail, calendar events, and contacts to Google Apps Mail.
You'll also need to know how to use a similar tool, Email Uploader, to move email from desktop software, like Outlook configured with POP accounts, to Google Apps Mail.