It will depend upon the specific spam filter used and how it is implemented by the email service provider; however, on the whole, a spam filter can offer effective spam protection. Many free webmail services now employ spam filters with a number of spam modules to detect spam on a number of levels, such as;
Most email providers automatically activate spam protection for every email account, and generally do not recommend deactivating a spam filter under any circumstance. The one problem with spam filters, is they can block automated email messages which the user wishes to receive: such as when they sign up for an account on a discussion forum, or have signed up for a newsletter. Due to these emails being automated, they often appear as spam to spam filters.
It should be noted that spam filters do not provide 100% spam protection, but, should, on the whole, block the majority of spam if they are kept up to date and installed correctly by the user/email provider. Spammers - the sophisticated ones - are always updating their techniques to circumvent the spam filters, so, there is always the risk a spam filter will not be effective for every type of spam.