To choose the best anti-spam email filter, you will first need to determine how much you want to spend on this type of computer program; there are some free options, as well as more aggressive options that are more expensive. It will also be necessary to verify that the anti-spam email filter is compatible with the email program you use, since not all programs will be compatible with all filters. In addition, these programs typically require an initial time investment as it "learns" what you consider to be spam and what is legitimate email.
Some anti-spam email filter programs will simply filter emails based on a constantly updated list of known spammers, while others will rely on a quick scan of the content of the email to pick up certain keywords that typically indicate spam. These are the simplest types of spam filters, and most email programs will have a built-in anti-spam email filter that operates under these parameters. Many people find that they still end up with a great deal of spam in their inboxes, however, which is when purchasing a commercial anti-spam email filter might be a good idea.
There are a number of different types of anti-spam email filters, some more aggressive than others. Two of the most aggressive spam blockers are those which use a "whitelist" to deliver emails to the inbox; the user will enter the email addresses for those people from whom he or she wants to receive email, and all others will be marked as spam. Some programs will also force each person who sends you an email to respond to an auto-generated message with a code to ensure that they are a real person sending the email and not a spammer. These types of programs are obviously quite effective, but they can lead to frequent missed emails.
The most popular type of anti-spam email filter is one based on a combination of the spammer blacklist and keyword restrictions mentioned above, as well as rules set by the user. These programs will generally take a few weeks to set up, as the program "learns" which messages you consider to be spam and which are legitimate. The user will go through email each day, marking certain ones as spam, and over time the program will be able to identify them without input from the user. It is important to regularly check your spam box to be sure that legitimate messages are not being sent there mistakenly.