Spam on iPhone

Why do some spam emails that are blocked on AT&T/Yahoo Mail get through to my iPhone? I checked the web site to see if these emails were in the AT&T/Yahoo Inbox and I found them in the Spam folder on the AT&T/Yahoo Mail web site. A dozen each day will still hit my iPhone email and I have to select them and use the Move to Junk. It still does not stop them from coming the next day. The AT&T/Yahoo Mail is set up as SMTP and also goes to my Outlook, which does not receive these emails. They only appear in on the iPhone. Thoughts?