There is no filter on the iPhone. Check if you email provider offers any server side filtering that you can set up from a web page? For example, my hosting company does support white-lists (allowed emails) and black-lists (blocked emails) but I have to set those up through my account web page. Those rules filter the emails as they come in to my account providers server, so they never arrive on my iPhone email client.
If your e-mail address is with iCloud, it does allow you to create some very basic "rules." When you phone sync's mail with iCloud, the rules should be applied. You have to created the rules on the iCloud Mail page, but they should then be reflected on your phone and other devices that get mail from iCloud.
I just tested the iCloud "rules" feature and yes, I can set up a rule in iCloud's Mail to intercept some mail and send it to a folder, and it works.
Here's the hitch, though. iCloud has had major problems with its push implementation, and one hitch is that when mail is sent to a folder, it is not pushed to the iPhone. It is pushed to the Mail client on the Mac, but not to Mail on the iPhone. You have to go to the folder manually on the iPhone and open it, and then it updates and shows whatever was sent. For Junk Mail that might actually be just fine, but for other mail sorting it is aggravating.
I love how Apple implemented different signatures for different accounts that was clearly needed. However, I still don't get why Apple hasn't had a feature to "sort by unread emails". I like to view my emails and then click unread after if they were very important. I only wish I could see the emails like that. I've talked to many friends who are entrepreneurs and have voiced the same thing.