Actually, it usually needs to be the other way around -- the
user needs the ability to override the site's filtering defaults.
I think that that should not be the case. Because if site
decides
to reject mails from AAA.com and if user wants to accept, I
think
site decision should be the first.
The solution is to have both. Our Mailserver has a server level script (run
at SMTP level), where your admin could reject mail with a site wide policy.
The end user has no say on the actions in that script. It also has an end
user sieve script where a user can file mail etc, but if that script takes
no actions, then it will end up in a server defined default mailbox script
that will take some default actions like spamtest/virustest etc.
So the end user can choose to opt out of some of the administrator defined
scripts, and not from others. That way you put the decision of where to
filter in the hands of the administrator.
Cheers,
Nigel