You may not realise it, but your email address is a valuable commodity. Active email addresses are bought and sold between advertisers and attackers alike, and there are various ways that they employ to gather these emails. Most simply, they trawl the web, looking for email addresses on web pages. But as most people are now wise to this and have removed their mail addresses wherever possible, they have found other ways of gathering addresses.

One method is to guess valid addresses, and send a speculative emails to test whether that address is active. Often, these mails have embedded HTML code which downloads an image or other file, which then alerts the attacker that the mail address is active.

It's a good idea to view emails in text mode, or, if your friends insist on using HTML in their mails, to view them without embedded images beign displayed; most email clients and webmail systems have options to do this.

When a gatherer has found a sufficient quantity of “live” addresses, they will sell them to advertisers or scammers who will send you those emails offering cut-price drugs, various bodily enhancements and the chance of a share in dodgy deals worth millions in Nigeria (or wherever). As with all types of unsolicited offer, almost all of these will be fraudulent, illegal, or at best unwise to follow up!