Hey,
You have to use IMAP for this, but ME Standard haven't it, you need to purchase ME Pro, Merak or Smarter Mail. Also as far as I know Horde can't work with ME Pro (ME bug), but ME Pro has it's own Webmail.
Does anyone know if SWsoft is planning to release a new Plesk for Windows version that will support saving of sent mail in Webmail without buying additional software?
As I know they are not going to supply Me pro. That is why the only way for you is to pirchase Me pro, Merak or SmarterMail. But Plesk does not support Merak 8.2