Beware social engineering

Be wary of certain types of sites (for instance greeting card sites etc.) - most require both your email address and the email address of the intended recipient.

This may sound like paranoia but the problem is on a less-than-reputable site you could have just handed over 2 good email addresses.

This is certainly not to say that all (or even many) sites are like this - but better to be wary that this can happen!