"Tell a friend" forms - Worth the risk?

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" method 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?

I've not heard of them being blamed as spam before (although I've seen people voice concerns). State in the body of the email that:

1. the email is sent by their friend to tell them about the site;
2. you keep no record of email addresses that the tell-a-friend messages are sent to;
3. if they consider it spam, they should email you with the body of the email so you can prevent the original sender from using your system again (track IPs and IDs in a database for this).

Then, when that's done, be sure to keep a database log of what the friends are actually saying. Not only will it help prevent abuse, but will also be a nice way of getting feedback ;)

Could also require the sender to put in their name/email address so there is something the receiver would recognise, this is obviously prone to abuse/misuse, but just a thought.

You could also use an independent sending thing like opt-media.

I'd ask your host, or just tell them what your doing so they don't interpret it as spam.

What I would do is first get the sender's email address. Then tell them that a link has been mailed to their email address and that, when it's clicked, it will send the mail on to their friend.

That way you can confirm the sender's email address and you can also send the mail as being "FROM: " their email address, so the friend is more likely to accept the mail as something friendly and not as spam. It also makes it harder for malicious users to abuse the service.

Thanks for the input everyone. I guess what I'm most concerned about is that no matter how much trouble I go through to stress that a friend sent the email, the email will be originating from my server and tracable back to my IP address. All it takes is one person with a trigger happy "report spam" finger and I'm in a world of hurt. I guess Incka's suggestion of using a service like opt-media could take care of that.

I know that several of the people who replied to this use tell a friend on some of their sites, so the fact that no one posted any horror stories helps me feel better about it.