If you use the mail system to handle all of your email, while you are reading a message you can make one of two choices: "Allow sender" or "Block sender". These choices are right above the message, in the same colored block that contains the subject. If you receive a message that is not marked as spam that you don't want to receive again, just select "Block sender". If the message is tagged as spam but you don't want future messages tagged, just select "Allow sender".

If you don't use the web based mail system for all of your email, you will need to go to "Options" in the top bar to the right of the Mustang Internet Services logo. The option icon is an envelope with gears on it. Click there, then choose "Spam filters". Type in the email address, select "Whitelist from", "Blacklist from", or "Whitelist to", and then click "Add New Address".