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.

No, not really, we are using one the most expensive offerings in Switzerland. Renting dedi servers for spamming would be not only extremely expensive, they would kick you right away if they noticed. As i said we did nothing wrong, used monitoring and our invoice emails would sometimes end in the spamlist for microsoft, after trying to get it right i gave up and hooked mandrill.

if you are runnig some random internet forum, some part of your notifications ending in spam is not a big deal, but for us, invoices ending in spam box is not something you can ignore.

And in my experience the major services have unpredictable spam filter behavior.

I even saw Google being cute and assuming mail in Japanese was spam - because my profile is western.

So yeah. If your goal is 100% (not high 90s) deliverability - including the odd big attachement or foreign encoding mail - it's not trivial.

Thankfully I don't care that much if a recepient at Gmail or outlook don't get my mail;it's their fault. If it's a problem the recipient should move to a more reliable mail provider.

It's obviously not as easy for commercial/work mail...