First of all: It's a little unclear that when 2.4 is installed I had to manually install the hotfix as well (at least I had to, to get it to work).
Second: When a user want to answer a message and I have set the Captcha service to internal and the user fails to enter the valid captcha the reciever disapears, so when the user enters the correct captcha the second time the form is not valid, saying that the To: field is empty...
Otherwise: a great component!
