How frequently are you sending emails to your subscribers? The higher you push the email frequency, the more likely you are to earn negative feedback from your subscribers. Push this too high, and you run the risk of subscribers complaining to the FTC, regardless of whether they opted-in to your email or not.
Of course, the vast majority of the email subscribers will simply unsubscribe from your list if they’re receiving too many emails, but with the CAN-SPAM act in place, I expect to see an increasing number of email subscribers taking up their complaints with the FTC. The fact is, people are angry at spam, and if you give them a reason, they may just take out that anger on you.
These days, any frequency greater than one email a week is considered quite heavy. Of course, this depends on the industry you are in and the initial expectations you set with your subscribers during the subscription process. Just be aware that if you send more than one email a week to you subscribers on your list, you may be risking some backlash.