So I want to make a HTML news letter to be sent out. I want to make a "Forward to Friend" button, but how would I link that?
The short answer is that you can't. Email clients don't provide a way for emails to trigger their Forward functionality.
The closest you could come would be to provide a link to a form which asks people to give you email addresses belonging to third parties to which you could then send email. (If I was that third party, the email would be dropped directly in my spam bin, possibly after filling out the annoying form for reporting spammers that is available from the OIC)
The good news is that email clients have Forward functionality built in, so you don't need to reinvent the wheel.
I'd try to go with the server side script route.
With PHP, use the mail function and retrieve the TO:$person from a post variable that is sent.
You could link it like:
Click <a href="http://bmtk.net/forward.php?**uniqueid**">here</a> to forward this email to a friend!
The closest you can probably come to by using a direct hyperlink would be something like this:
mailto:[email protected]?subject=New+email&body=Blahblah
If the user tries to open this hyperlink, this should trigger the default email client of the user to pop up with a new email window where the recipient's address, subject line, and body are already filled in. I'm aware that this might not be the functionality that you're after.
Also, it is quite likely that this will not work with all email clients out there, and even if it did, it would be very cumbersome to append the whole email body to that hyperlink URL.
Therefore, I would also suggest you solve this problem either:
by letting the email reader directly use his email client's forward facility; or
direct the email reader to a web form where you let him/her supply the friend's email address, then generate the mail to be sent in a script on the server, and use an email server to directly send out mail.