You definitely want to watch the spam rate complaints of these emails vs. your normal sends. Companies like SailThru won't let you (or strongly discourage) using their mail server and IPs to send friend referral emails because they get dunned by ISPs from spam complaints on referral emails all the time.
You can always run this through a separate account or service on a different IP so that your marketing comms don't take any hits over complaints on your referral emails.
I'm not saying you will get them, but just a heads up that it's a common problem as you scale it.
Good point. I didn't get any complaints and my unsubscribe rate wasn't any higher than usual, but that might be because I sent the referral link as part of the regular Sidebar newsletter.
On the other hand, if I had sent out those links on their own, I can see how people would perceive it as spam and complain a lot more. Then again, I might also have had a higher conversion rate, who knows… (but I'm not taking the risk of potentially alienating my subscribers to find out!)
You can always run this through a separate account or service on a different IP so that your marketing comms don't take any hits over complaints on your referral emails.
I'm not saying you will get them, but just a heads up that it's a common problem as you scale it.