Hi,
I think that smtp auth in the 1.8 series rubies has a small bug. smtp.rb
checks respond_to?(auth_method), but the relevant auth methods are
now private, and so the check always returns false, raising an ArgumentError
since the "includePriv" argument of respond_to? is not being set.

In 1.6.8, these methods were public, so that the check worked ok.
Below is a small patch, in case that makes things any
easier.

--- smtp.rb	2003-07-29 08:36:28.000000000 -0700
+++ smpt.rb.new	2003-08-02 02:43:00.000000000 -0700
@@ -520,7 +520,7 @@
                       unless user and secret
       auth_method = "auth_#{authtype || 'cram_md5'}"
       raise ArgumentError, "wrong auth type #{authtype}"\
-                      unless respond_to?(auth_method)
+                      unless respond_to?(auth_method, true)
     end
 
     def authenticate( user, secret, authtype )