If you’d like to check out ConvertKit, my referral link is here [Full Disclosure: I’m a compensated advisor and an affiliate for ConvertKit], and if you have any questions about ConvertKit, both Nathan and I will be actively popping in and out of this post to reply, however we’re currently at the LeadPages conference in Minneapolis so we may be slow to reply at first.
I am not signing up for an account so I don't know exactly how the API works, but if I were setting up an iPhone App to work with this, I would use the rails plugin. Rails gives you JSON communication for free (IE no extra work required) http://guides.rubyonrails.org/layouts_and_rendering.html see section 2.2.9. Then use on IOS app use something like: http://code.google.com/p/json-framework/ to interact with that.
“As my list has grown, I’ve just remained focused on the next metric right ahead of me. First it was just to hit 200 subscribers, then 300 subscribers, then 400 subscribes. Eventually the goal was 1,000 subscribers, then 2,000 etc. etc. As I’ve gone past 10,000 and 20,000 and beyond I’ve actually started to focus more not on the sheer number but on engagement.” – John Corcoran, Smart Business Revolution
Finally, you can optionally include “automations” with your broadcast. Automations are a simple way to automatically add or remove tags when your subscribers do certain actions, like open the email or click on specific links. Adding and removing tags are important when creating segments, or subscriber groups, and triggering automated email campaigns.