I'm a sys admin of Microsoft Exchange 2000 and have 3 questions about
using digital certificate (X509v3):

1. How can Outlook 2000 users view the digital certificate on GAL ?
... similar to Outlook Contact which has a Certificates tab. I know
the certificate has been published on GAL because I can encrypt a
message to the GAL recipient.

2. Does GAL store multiple digital certificate (x509v3)? 

  2a. If yes, how can the sender pick the right certificate of
recipient for encryption ?
  2b. If no, how can the recipient decrypt the old message which
certificate is expired ?

3. Any good reference (site or book) on this topic ?

P.S. I'm not using Microsoft Cert server.