Gmail bouncing postfix mail [duplicate]

The tutorial is great (I've followed it myself) but there are a few significant things missing, most of them DNS-related. Take a look here: http://blog.codinghorror.com/so-youd-like-to-send-some-email-through-code/

I can not stress this enough: you need a reverse DNS record (PTR). Most email providers will just drop messages from hosts that have no PTR. Just fix this first, there's no point working on the rest if you don't have a PTR.

A correct SPF record is not needed per se for delivery, but it will HUGELY improve your messages' chances of not ending up in the spam folder.

Accordingly, you'll need to configure DKIM. After that, add your domain on a public DNS whitelist.

You'll still end up in some spam folders (if I remember correctly outlook.com seems that doesn't support DKIM correctly), but less often than previously :)