Google Apps Directory Sync for Postini Services Administration Guide - Manual (Page 52)

Configuration 55
Mailing lists are often handled separately, because if a mailing list is added as a
user, all recipients of the mailing list will receive notifications. This can cause
confusion. To avoid this problem, Google Apps Directory Sync can import mailing
lists as aliases to a user you specify.
This section is optional. Fill out the information in this section if you wish to import
mailing lists as aliases to a central user in the message security service.