By the way, speaking of incentives Zapier's own referral program is very well done. It's quite fun trying to earn all the extra tasks, and I'm almost there!
I'm reposting under the assumption that the flagging was a false positive, because I can't see anything wrong with the link itself.
Of course if I did do something wrong, I would love to know what it is so I can avoid doing it again.
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.
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!)
A quick anecdote: While reading this article I noticed that I don't remember getting my referral link from Sidebar. I searched my Gmail for "in:spam sidebar" and there it was - the only Sidebar email which Gmail has ever flagged as spam.
I know MailChimp has tools for testing the spamminess of a campaign and this message probably passed all of them since there's nothing particularly spammy about it. Maybe it was random or maybe the referral link was just enough to trigger the spam filter in (my) Gmail.
If the open rate for the MailChimp campaign wasn't particularly low spam filtering wasn't probably a significant factor in the overall conversion rate.
And you're right, the open rate is much lower than usual (19% vs 63% for the one before). I didn't noticed it before because the daily edition containing the same referral link that I sent on the same day went through fine.
This happens from time to time and I'm not sure what I can do about it.
I honestly don't understand how Gmail's spam filter can be so "dumb" (at least from a human point of view). You'd think that if someone open, reads, and clicks emails from a particular address regularly, the filter would be smart enough to know that there's a very high probability it's not spam.
I guess the least we can do is help Google tune their filters and periodically mark all false positives as "not spam".
Additionally one might go as far to suggest false positives are more problematic than the occasional spam mail.
For example, one Sidebar subscriber ended up referring 8 new people! So it can be a good way to identify your biggest fans.
Feedburner has very simple implementation but it seems like there's a good chance it will be retired soon so I was hoping for another option.
They offer free blocks of messages if you're a Mail Chimp customer.