I think it depends on the email client that person uses, some may not have this functionality.
The header you want to send with the email is:
As I said, this may not work 100% of the time, but try it out and see what happens.
$headers = 'From: email@example.com' . "\r\n" .
mail('firstname.lastname@example.org', 'Subject', 'Message', $headers);
As for the spam thing, that's something completely out of your control, and there's no way of knowing whether or not your email will go into someone's junk folder.
There are sites out there that can check your emails (at a cost) for "spammyness" if you're concerned about that.
Can't think of any off the top of my head, but google is your friend.