To manage your Antispam settings, go to https://antispam.letu.edu/. There it will take you to this login page:

Use your Letnet username and password to login. It will then redirect you to your home page, showing you your pending spam and actions you can take (or other views depending on homepage customization). To view all of your pending messages (messages that could be spam or legitimate messages as well), go to "Trap contents" in the to menu bar, then "Pending" in the sub-menu.

Here you can see all your pending messages, the senders, and more information such as IP addresses and the actual spam score. The higher the score, the more likely it is to be spam (the default threshold is 5.0).

Under the "Status and Action" column, all messages should be marked as "Pending."

In the pull-down menu for each message (shown in the picture above), there are the following available actions:
Do Nothing
Accept Message
Reject Message
Silently Discard
Blacklist Sender
Whitelist Sender
Blacklist Domain
Whitelist Domain
Blacklist Network
Whitelist Network

Here are the following explanations for all of the available actions:

Do Nothing: The mail remains in your "Pending" section and stays there for 30 days. After 30 days, the message is automatically discarded by the Antispam server.

Accept Message: This passes the message to your inbox.

Reject Message: This causes the Antispam server to actively reject the message back to the sender. Because of the method used to discard messages, this action will likely decrease the amount of spam you receive from that sender.

Silently Discard: This causes the Antispam server to simply delete the message, never sending a notification back to the sender. This is the most simple method of getting rid of spam.

Blacklist Sender: This rejects the message and causes the Antispam server to permanently reject any message that comes from that specific sender (i.e.mta4.emailinfo2.bestbuy.com).

Whitelist Sender: This passes the message to your inbox along with any other message that comes form that particular sender permanently (i.e.mta4.emailinfo2.bestbuy.com).

Blacklist/Whitelist Domain: Same as above, but applies to the entire domain of senders (i.e. all mail servers that are part of bestbuy.com). This is generally discouraged, but can be used if receiving massive amounts of mail from one particular website that still gets through filtering.

Blacklist/Whitelist Network: Same as above, but applies to the entire range of IP addresses. This option is HIGHLY discouraged, as it can affect many different mail senders other than the particular sender your choice applies to.

Blacklisting and Whitelisting are permanent actions and are always discouraged. The Antispam filter does a highly effective job at learning specific senders and types of spam, and will train itself to filter the messages more effectively.

If you have blacklisted or whitelisted senders, domains, or networks, you can modify those settings by navigating to the "Rules" menu and "Blacklisted Recipients" or "Valid Recipients" sub-menu.

In this window, you can add or delete whitelisted/blacklisted senders, domains, and networks.

Be sure to click on "Submit Changes" when you finish checking for deletion and/or adding recipients.