"Tell a friend" forms - Worth the risk?

[Westech]

I'm considering putting a "tell a friend" type of form on a couple of my sites. The form would let visitors enter a friend's email address and a short note and my server would then send the friend an email with the note and a link to my site. I'm a little hesitant to do so because the users who recieve these emails may report them as spam and I don't want to risk having my IP blacklisted or my server terminiated.

I'd like to hear from those of you who use the "tell a friend" or technique on your webpages or forums. Do your visitors use it often? Have you ever run into problems with it? Do you have any advice on how I should implement it to be as effective as possible with as little risk as possible?