Here is a revised mailclient.el and a patch for sendmail.el to enable  
it.
It contains some changes to allow for the e-mail body to be placed on  
the clipboard in order to accomodate w32 systems that apparently  
can't handle longer URLs. Lennart Borgman had suggested that; I have  
implemented it using clipboard-kill-ring-save instead of some OS- 
specific w32-* function, so it'll work on all systems.

Please add it to the trunk if you think it's ok now.

==


*** sendmail.el 6 Aug 2005 18:54:05 -0000       1.287
--- sendmail.el 14 Aug 2005 18:57:45 -0000
***************
*** 120,126 ****

   ;; Useful to set in site-init.el
   ;;;###autoload
! (defcustom send-mail-function 'sendmail-send-it
     "Function to call to send the current buffer as mail.
   The headers should be delimited by a line which is
   not a valid RFC822 header or continuation line,
--- 120,129 ----

   ;; Useful to set in site-init.el
   ;;;###autoload
! (defcustom send-mail-function
!   (if (memq system-type '(darwin windows-nt))
!       'mailclient-send-it
!     'sendmail-send-it)
     "Function to call to send the current buffer as mail.
   The headers should be delimited by a line which is
   not a valid RFC822 header or continuation line,
***************
*** 130,135 ****
--- 133,139 ----
     :type '(radio (function-item sendmail-send-it :tag "Use Sendmail  
package")
                 (function-item smtpmail-send-it :tag "Use SMTPmail  
package")
                 (function-item feedmail-send-it :tag "Use Feedmail  
package")
+               (function-item mailclient-send-it :tag "Use  
Mailclient package")
                 function)
     :group 'sendmail)