Migrated to Google Apps
All email in top-level folders is migrated. A top-level folder is any folder that resides at the same level as your Inbox folder.
You have the option to exclude specific top- level folders from migration. For more information, see “Step 3” on page 43.
Email state (read vs. unread)
Folders and nested subfolders (Labels in Gmail)
Folders map to labels in Google Mail.
You can elect to omit top-level folders from migration. If you omit a folder, its subfolders are also omitted.
There is a 255 character limit per label. If you’re migrating a folder with a title that’s over 255 characters, the mail will still migrate, but it will be put with the label prior to this.
Migration is optional.
Migration is optional. Deleted items are labeled Trash in Google Mail.
Public folders (Microsoft® Exchange only)
Google Apps does not include public folders.
Messages larger than 25 MB
Messages whose total size exceeds 25 MB (message plus attachments) aren’t allowed in Gmail.
Most attachments are migrated. See below for exceptions.
Executable files in compressed attachments
These types of attachments aren’t allowed in Gmail.
Posts in mailbox folders (Microsoft® Exchange only)
Not available in Gmail.
Importance levels (marking messages high, low priority)
Not imported into Gmail.
(Microsoft® Exchange only)
Flags appear as stars in Gmail.
Rules (Microsoft Exchange only)
Rules (both server and client) aren’t migrated. But you can create equivalent filters in Gmail.
Google Apps Migration for Microsoft
Exchange Administration Guide