Nice to hear it's working..
Though this tells us dovecot needs correct user AND group rights to function. (php safe_mode like thingy)
I'm not using dovecot at all, so I can't tell you if that's the case, this assumption is based on the response I get from you
Maybe Till can shed some more light on this?
edit: remember to manually set g+x on the newly created email domain! (or patch the php code that creates them ;-))