Filtering addresses on outgoing messages

[erme2]

Hi all,
I'd like to write a zimlet to filter outgoing messages. When the user click on the "send" button, before the message in sent I'd like:
1 - catch the addresses
2 - for each address {

if (the address is in a specific address book) { it' ok!}
else {delete it!}

}
3 - if the message still have one or more addresses we can send the mail

Where I can start from?
thanks

[erme2]

ok,
I started here:

mynameZimlet.prototype.emailErrorCheck =
function(mail, boolAndErrorMsgArray) {
    mail.clearAddresses();
    var errParams = {
        hasError:true,
        errorMsg: "NO!",
        zimletName:AttachAlertZimlet.ZIMLET_NAME
    };
    return boolAndErrorMsgArray.push(errParams);
}

It shoud clear the address and stop the mail, but it's not working...
What I miss?

Thanks