The best method is to have them request a screener after they have seen a trailer and film info from your website. A direct link to the trailer in the initial email starts off the process. Once a screener is requested, then it does not need to in any special packaging and it will no longer be "unsolicited".
Well whatever "non- solicited" method you use... emailing or calling first is highly recommended otherwise your screener might not even get viewed and you'll have no one to contact a few weeks later to check on the progress. Guaranteed they won't be hounding you to tell you what they thought of your film. You're going to need to call or email repeatedly to hear something along the lines of "Yeah we really liked your film and we're sending it around the table before making a decision" or "Yeah we really liked your film but without a name we can't market it"