Please update roundcubemail to 0.5: http://www.roundcube.net/download 0.5 seems to be fast: http://www.happyassassin.net/2011/01/27/roundcube-0-5-really-good/
Net_IDNA2 is needed in 0.5.1. roundcubemail 0.5.1 seems to work great in local testing.
0.5.1 is in rawhide, and will be in F15.