Will Postini be white-listed on the CP boxes?
Not as such, no. To begin with the only whitelisting we'll have will be that on the sunmxcore's. This is where the subject line tagging is actually applied (regardless as to which anti-spam mechanism identified the email as spam) and is one way we'll be able to fix false positives.
Can I enter the trial if my xxxx.plus.com mail is turned off, and all my mail arrives via a domain name?
Yes.
Or have I got the wrong end of the stick, Postini does not stop this?
Postini should reduce the volume of spam you receive, yes.
I assume that if we sign up to this trial the spam settings for our email accounts remains the same. i.e in my case spam goes to the spam folder but does not get deleted.
Yes that's right, although Postini should stop more junk from actually getting through in the first place. This level of 'edge' blocking is applied irrespective of your spam settings and how noticeable it will be will depend on the amount of spam you get in the first place.