Filters, RBL's, etc. like Spam Assassin and the like they use.
If you can, always move "wanted" messages from your spam folder to your inbox; this will help "whitelist" the sending address.
For "unwanted" messages, select them and then click the spam button. This will help to "blacklist" the sending address domains. If the message appears legit, but you don't want them anymore, then click their "unsubscribe" link. The message may need to be transferred to your inbox for the link to work. Give it time and your mailbox will begin to "remember".
Hope that helps...
