I've been told that if you receive an e-mail that tells you to respond if you want to be removed from their mailing list (such as a porn site or other undesirable ads), you should not respond. If you do, the site will sell your e-mail address to other related sites, even though they take you off their own. Is this for real or just another urban myth?
This one is indeed for real, not a myth. The rule of thumb is this: if the address of the sender of a message is forged, then nothing they say should be trusted. LEGITIMATE e-businesses use legitimate e-mail addresses, not forged ones. Legitimate e-businesses are also a vanishingly small minority, and most have the sense not to send unsolicited bulk e-mail. I get bulk mail from a number of companies and services (Discover Card, Pacific Bell Telephone, etc.), and they have a “to be removed from our mailing list” clause, but these are mailing lists to which I actively subscribed (called “opt-in” lists). Generally, you can trust opt-in lists not to sell your address without your approval. But most of those unsolicited messages you get with instructions on how to be removed from their pestiferous list (called “opt-out”) are using responses to their ads to create lists of known valid addresses which they then sell to other bulk e-mailers to help recover their costs (sort of a pyramid scheme, in a way).
You can find people selling e-mail address lists all over the place, and you will most definitely get on those lists if you respond to unsolicited bulk e-mails trying to get removed. The only effective way to prevent someone from sending you their unwanted ads is to track who they actually are (not who they claim to be), and file a complaint directly with their internet service provider (ISP). Since bulk e-mail is a violation of most service contracts, a properly documented complaint to an honest ISP will cost the spammer his/her account. Not only will you never receive spam from that account again, thousands of other unwilling victims will be spared a repeat visit as well. The second rule of thumb, therefore, is “never hit delete, never hit reply”–trace the sucker, and hit them where it hurts. Here
Send questions to Cecil via firstname.lastname@example.org.