I would not recommend to go this way. It does not make sense to develop an XF-addon, which is again requiring to work with something else which costs money. People who want that, can go directly to these providers. They do not need an addon.
What the XF-community is desperately looking for is a very simple bounced email management while using ONLY the XF-forum, no other third party payed service.
so your newsletter addon is the right start in this direction. If you absolutely wnat to and have too much time, then you can add lateron support for third party services. But an addon which just relies on that firsthand, will not be downloaded many times. Money rules here.
Look at the following email bounced management addon for vb, to get an idea, what the basics are you absolutely have to have:
http://www.vbulletin.org/forum/showthread.php?t=138884
if you think about bounced email management, you have always 2 major drivers:
1. emails that are no longer valid or mailbox full and come back with an error message, or even worse do not come back
2. Spam management of webhoster, who look how many emails you send to them per minute and per hour, to how many emails from the same domain etc pp. Keyword is "throtteling" etc.
#2 is already advanced and you should focus first on #1. If you want to get ideas how to do this with email throttling, look at
www.phplist.com. THis is by far the best and most known free newletter available out there. Very professional, with settings for everything incl. a very advanced email bounced management. But there does not exist (yet) a bridge between XF and phplist.
It would be an interesting idea to link them somehow, so that mail addresses ar in real time the same between the two and bounced emails in phplist would take the appropriate actions in realtime within XF db like the EZbouncemanagement script.
