The Preferences - Whitelist / Blacklist page lets you determine which mail you do or do not want to recieve.

The blacklist is a list of senders from whom you never want to receive messages. The Barracuda Spam Firewall immediately discards messages from senders on your blacklist. These messages are not tagged or quarantined and cannot be recoverd. The sender does not receive a notice that the message was deleted and neither do you.

The only time a blacklisted email address is delivered is if the same email address also appears in your whitelist.