user (both prevalidated and validated) accounts are the only login
     accounts subject to automated purges.  These purges occur weekly and
     are based on your last UNIX login time if you are validated, or the
     number of days since your account was created if you are prevalidated.

     ARPA members are exempt from inactivity purges, though an ARPA member
     may lose their membership by violating the AUP.

     Prevalidated accounts are automatically purged whether they are in
     use or not approximately 200 days of their creation.

     Validated (user) accounts are purged typically after 3 months or 6
     months of inactivity.  It is variable because it is dependent upon
     system resource use.  Meaning, if we need more user ids or disk space,
     then inactive validated users will be purged.

     Keep in mind that using SDF like its 'yahoo' or 'hotmail' doesn't 
     constitute as a UNIX login.  You will have to login to the shell 
     account regularly to avoid being flagged as an inactive account.