When done once in a while, this isn’t a problem. But when used habitually, eventually the list will get to someone who has no qualms about adding these email addresses to the database of people that s/he uses to hawk viagra and other crap.
A simple way of protecting the people on your list from this abuse is to send the message ‘To:’ yourself, and then ‘BCC:’ (‘blind carbon copy’) to the addresses on your list. If you do this, the recipients will only see a single address – yours. All of the other addresses will be completely invisible to each recipient (the email system will have stripped them out before the email arrives).