Theresa's boss gets a lot of spam and wants a spam blocker. Leo advises against them, especially for a business because they cause people to jump through hoops. Spam blockers send emails back to the sender, asking them to validate that they are not spam. This will let spammers know that this is a good address, so once she starts using it, she would never be able to stop. Leo recommends using gmail, because it is a brilliant anti-spam solution. She could have her email forwarded to the gmail account.
If she still really wants a better spam filter, she could try OnlyMyEmail.com.