Google Message Security for Google Apps Administration Guide - Manual (Page 375)

Message Log Search 375
User ID Description: Unique number from the message
security service that identifies the sender of an
outbound message or the recipient of an inbound
message. A user’s primary email address can be
changed, but its ID always remains the same.
A User ID is useful for tracking a user even when
they have multiple aliases. To locate a User ID, log in
to the Administration Console. Go to Orgs and
Users > Users, and then click the relevant user to
open the User Overview page. The User ID is
displayed in the Summary box on the right side of the
page.
Value: system number followed by the user ID.
Example: 7-206340783
User Type Indicates whether a sender or recipient is a
registered user, unregistered user, or catchall user (a
legacy feature).
Virus Name Name of any found virus
Field Description and Values