First, you need to make sure curl http://127.0.0.1:2002/abc works locally.
Then you can add the rewrite inside of the rewrite tab.
REWRITERULE ^(.*)$ HTTP://webmail /$1 [P,L,E=PROXY-HOST:WWW.EXAMPLE.COM]
If it works, then you can add a rewritecond for the "webmail".
Note, please...