There is a significant market for commercial spam-filtering services. Typically, these are suitable for individuals and small companies that access email via POP3. The different approaches taken by providers are described below.
In this approach, the users give their details to the anti-spam service provider. This information includes their ISP details, username, and password. The anti-spam service company then regularly connects to the ISP on behalf of the user and collects all the emails. The spam is filtered out and ham email is forwarded to another POP3 account.
One obvious disadvantage of this approach is that the account username and password have to be passed to another party. It is not wise to give out this sensitive information.
Another disadvantage is that any emails wrongly identified as spam may be lost forever. If the anti-spam service provider provides an archive of spams emails, they can be reviewed and any wrongly identified emails...