Spam folder question
 

I can't figure out why yahoo sometimes shoves email into the spam/bulk folder.

I have a form send a confirming email to people signing up for stuff.

In some of my accounts (as tests) it goes into the yahoo bulk folder, in some it does not. Thay all have the same setup regarding spam controls and filters. They're basically generic untouched accounts.

Anyone have any ideas what yahoo uses as logic??