Google Message Security for Google Apps Administration Guide - Manual (Page 163)

Quarantine Summary & Notifications 163
We recommend that you set notifications for user organizations (though they
can be set at the account and email config levels).
2. On the Organization Management page, under Organization Settings, click
Notifications.
3. Click the Spam link.
4. Enter a URL for the logo image you want to use, and enter text for the
message body.
• Logo: Add a URL where the logo can be found on a public web site. The
recommend logo size is 120w x 34h pixels.
• Message Text: The maximum number of characters is 500, and the
message text does not support HTML.
See “About Customizing User Notifications” on page 383 for more information on
this.