One anecdote against another. My suspicion is that you were in a "bad neighbourhood" - ie. using a hosting or email service that didn't keep their IP's squeaky

One anecdote against another. My suspicion is that you were in a "bad neighbourhood" - ie. using a hosting or email service that didn't keep their IP's squeaky clean. AWS/EC2 is a common host among the HN crowd but is one of the worse places to host a own mail server at for this reason - they'd also rather upsell you SES than fix the EC2 IP ranges' reputation. Microsoft is the pickiest email provider to get good delivery rates to - they require a gradual volume ramp-up over 1-4 weeks on a fresh IP to see it as good, in my experience, and SNDS is something to keep a close eye on during that time - it clearly indicates your IP's reputation with them and how much mail they see from it. That number being higher than expected plus the sample emails they provide may indicate something going wrong - through this, I once discovered an obscure feature in a software package I used that allowed visitors to send emails to arbitrary addresses. Mandrill has better delivery than the other email services I've tried but is cripplingly expensive for a hobby project that happens to have a nontrivial userbase.