Thanks for the explanation. OK, some things are clearer
now. However, it seems that I could no longer receive
email directly, i.e. have an MX record pointing to my IP
address or, if I did, I would have to reject stuff from
non-trusted senders. Right? (This is certainly possible.
In fact, I could do it now. But then I have to have someone
not just carry the MX record, but route all of my mail through
his server.) Also, would it be possible for a dynamic IP address
to get on the trusted list? It seems that these folks will
always be on the non-trusted list, unless an ISP wants to vouch
for all of his addresses, in which case he would probably require
customers to send email through his own SMTP relay server.