Redirect existing mail address

In windows client

 1. Setting -> Mail -> Mail boxes -> doubleclick user to be redirected -> set combobox Active to No.

 2. Scribe down (to notepad or piece of paper) users mail (just the name - part before @) and its aliases.

 3. Setting -> Mail -> Mail boxes -> doubleclick user to redirect to -> push "+" button in Aliases section -> fill up user aliases which you were scribbing down