Newsletters is a great way let your members to see adsense advertisement.
Adsense forbid email, but seems fine on APP now.
They even offer Admob or APPs.

As we know, APP can push SMS to users, even better than email.

The difference between email and app sms:
Email: 1.mail or webmail software-> 2. browse email (Adsense forbid) -> 3. click a link on email to the webpage with Adsense
ThreeTZ SMS: 1. Smartphone sms -> 2.Browse webpage with Adsense on ThreeTZ APP browser
So, using free SMS APP as ThreeTZ can shorten the steps for users to see webpage with Adsense.
Also SMS is more easier to reach users then emails.

So, I can push SMS to my members who use ThreeTZ app as 2-step verification or CAPTCHA,
then display webpage with Adsense on APP browser to them.

But it's too troublesome to make my own app for both Android and iOS.

I searched a while and find an new app, ThreeTZ OTP that allows developers/webmasters to push free sms to their own members who use the app.

Since it's a free one time password tool, I can ask my users to download it as a CAPTCHA alternative solution.

For now, I can push my webpage with ThreeTZ developer panel as newsletters but much higher reach rate.

Here is the page for webmaster
http://www.threetz.com/developers/

My case:
I have intergrated the app with my free classifieds: http://www.cathaylist.com
Welcome to post free ads on my site, then you can see it.
After you post a free ads with ThreeTZ free OTP APP on my free classifieds.

If you use the same classifieds script as I am using now, pm me.
I can give you the part of script I integrated with.