I can think of two ways.
Setup two mailboxes, and give them "public address" and "private address". For mailbox "public address" add a keyword filter to forward to "private address" and then configure mailbox "public address" to remove unread messages. Then you just have to login to "private address" to read messages.
The DreamHost Junk Filter implements SpamAssasin and thus when logging into the DreamHost install of Squirrelmail you can modify a whitelist/blacklist.
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