Various anti-spam techniques are used to prevent email spam (unsolicited bulk email). No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) as opposed to not rejecting all spam email (false negatives) – and the associated costs in time, effort, and cost of wrongfully obstructing good mail. Anti-spam techniques can be broken into four broad categories: those that require actions by individuals, those that can be automated by email administrators, those that can be automated by email senders and those employed by researchers and law enforcement officials. End-user techniques There are a number of techniques that individuals can use to restrict the availability of their email addresses, with the goal of reducing their chance of receiving spam. Discretion Sharing an email address only among a limited group of correspondents is one way to limit the chance that the address will be "harvested" and targeted by spam. Similarly, when forwarding messages to a number of recipients who don't know one another, recipient addresses can be put in the "bcc: field" so that each recipient does not get a list of the other recipients' email addresses. Address munging Email addresses posted on webpages, Usenet or chat rooms are vulnerable to e-mail address harvesting. Address munging is the practice of disguising an e-mail address to prevent it from being automatically collected in this way, but still allow a human reader to reconstruct the original: an email address such as, "no-one@example.com", might be written as "no-one at example dot com", for instance. A related technique is to display all or part of the email address as an image, or as jumbled text with the order of characters restored using CSS. Avoid responding to spam A common piece of advice is to not to reply to spam messages as spammers may simply regard responses as confirmation that an email address is valid. Similarly, many spam messages contain web links or addresses which the user is directed to follow to be removed from the spammer's mailing list – and these should be treated as dangerous. In any case, sender addresses are often forged in spam messages, so that responding to spam may result in failed deliveries – or may reach completely innocent third parties. Contact forms Businesses and individuals sometimes avoid publicising an email address by asking for contact to come via a "contact form"