Distributed authentication. Which one?

[Francisco Reyes]

I inherited a number of machines and now have to deal with maintaining a 
small set of users accros 20 machines.

Although the list of users is very small, will alsolike to use the 
same authentication scheme for handling authentication against virtual mail 
accounts.

The setup:
mail provider with  SMTP authentication.

Currently using MySQL accross different machines, but not shared. Would 
like to have a single way to authenticate all email users accross all 
machines.

Preferably a distributed system which can run on top of PostgreSQL.


I have read of the following: NIS, LDAP and Kerberos.
Security is obviously a concern.