Why does my members get my site emails sent to there junk folder?
I have the same question!
A lot of spam filters see site emails as junk. I'm not sure why, and I don't think there's anything you can do about it other than to make sure your members know to check their SPAM folders and mark emails from your site as 'Safe' or 'Not Junk'.
Run the test outlined on this page:
http://wiki.mybboard.net/index.php/Help:Mail
If that email is sent without warnings then the problem is at the end of the receiver, probably in the form of a firewall or spam filter.
Also try sending via SMTP, this helps some times.