Bounce back letters are nothing but running in circles and a wasted effort. What I'm getting now are people who are interested in my offer and not just someone trying to sell me. And I don't have to worry so much about being accused of spamming. It's great, if you're posting ads where you can't use a web page address and only an email address. Frankly, I'd rather have them go to my web page.
Pictures speak a thousand words.
Another form of spamming. And the ones that are being sent to me drive me nuts. Send one follow-up if you must, but you take the risk of being accused of spam. And the potential risk of irritating a possible future customer. You never know when they'll show up again and your follow-ups could come back to haunt you.