Stormguard and Iza are correct in a technical way, this may be hard to understand by people without computer background. I would like to help explain in the other way.
Once your email address is on a spammer's list, it can be very hard to get off, especially if he's selling it. Here are some tips on how to stop spam before it starts.
Reduce your exposure to ``harvesting''
Don't display your email address(es) in public more than necessary, at least not in a form that's easy prey for scavenger bots (programs that spammers run to ``harvest'' email addresses).
If you publish web pages, don't put your address in a click-to-email link (HTML's HREF MAILTO). This means people have to cut-and-paste your address rather than emailing your with a single click, but it can save you a lot of spam.
Rather than putting your email address on every page of your site, it may be better to have it on only one page, with links to it. Many spammers don't even bother to remove duplicates from their lists.
If your address appears on a very well-indexed site you might want to use one of the many common tactics to disguise it from scavenger bots, (such as splitting the components of the address as Junkbusters does on its feedback page), or inserting HTML code in the middle of the address (as JUNKBUSTERS SPAMOFF does).
If you post to Usenet or chat, consider disguising your address. There are dozens of ways of doing this so that humans who really want to contact you can figure out how to do so: look at a few postings and choose one you like. We have heard reports that some harvesters are already wise to addresses such as me@nospam.myisp.com so try a variation on the nospam. There are plenty of adjectives that could be added. Of course almost any method can be thwarted by sufficiently intelligent scavenger bots, but most of them aren't very smart.
An alternative is getting a free forwarding address from companies such as Bigfoot.com or NetAddress.com. Many of them include free filtering facilities.
Choosing an alias or email address that begins with a letter late in the alphabet may reduce the amount of spam you get, because many lists are sold sorted in alphabetical order, and spamming sessions are often terminated before they complete.
Lucent's LPWA has an innovative system for producing ``revokable'' email aliases that can be used when submitting your address in forms on a web site.
Check your browsing isn't giving you away - In a very small number of cases, your email address may be discovered by a web site you visit. This has occurred through bugs in places such as Web-based email services. Early versions of some browsers gave away email addresses routinely. A quick and easy way to check for this is to visit our privacy check page that displays these headers. If your address is being disclosed, reconfigure your browser or use a free product such as Internet Junkbuster to block it.
Check that your ISP or company isn't running the identd demon, a background program that Web servers can ask for your user ID while you wait for a page. It was originally intended to restrict access to authorized users, but spamming sites can use it to guess your email address. Our test lets you quickly find out whether your identity is being revealed in this way.
Some larger companies sell or give away email addresses. Most have policies and terms that prohibit use of their information for spamming, but this doesn't seem to stop some spammers.
You can tell email lookup services such as those listed in Yahoo to remove your name and email address.
Some of the giant companies that sell mailing lists to catalog companies are now also selling email addresses. Most of these don't have Web-based ``opt-out'' forms, but JUNKBUSTERS DECLARATION gives you an easy way to draft physical letters to them.
The threat of litigation is the basic idea behind many anti-spam tactics. A carefully-worded ``offer'' to receive spam for a fee of $10 per piece is provided in JUNKBUSTERS SPAMOFF for anyone to use in reply to spammers. You can even publish it on your Web space if you wish. Ten dollars is an amount which can be the subject of a suit in small claims court, giving everyone a credible way to sue spammers. Other commonly used legal threats include claims that spam is an unsolicited fax under the Telephone Consumer Protection Act, and statements that spam will be interpreted as an order for proofreading services.