[JonLim]

I'm the Product Manager for PostageApp (www.postageapp.com) and we built our email delivery engine from the ground up (rather than using existing solutions like Port25) and we learned a huge deal about getting emails into inboxes.

This article is a fantastic start and will definitely help you out with delivering email, but there are some minor nuances among different SMTP servers and email providers that are just so quirky that you really can't account for 100% of everything.

Organic growth of your email volume to users who genuinely want your emails is a great way to make sure you stay in their good books, as well as observing some SPAMAssassin rules and auditing your subject line and content to not seem spammy.

It's a huge and complicated matter (getting emails into inboxes) and it's not documented, thankfully we are getting a better and better idea of how to do it.