The top checkbox allows you to choose whether or not to scan Inbound e-mail messages. Since e-mail is one of the primary means by which viruses and worms get into your system, it is important that you leave this checkbox checked.
Under that checkbox are two radio buttons which allow you to decide how to handle detected threats. There is an option that says Prompt me when an attachment needs to be cleaned, but that may simply result in a lot of prompts from your antivirus software that most people won't really know what to do with. I recommend you leave the top choice, Automatically clean infected attachments, selected.
At the bottom is a checkbox to choose whether or not to scan Outbound e-mail messages. If your computer never became infected then obviously you would not have any infected outbound communications. However, it is a good idea to leave this option checked so that you might be alerted if your system does become infected and starts sending infected email attachments out to others.