Where did you get the idea of $100/mo? SendGrid has 100 emails per day free tier and PostMark has 100 free emails per month and basic plan is $15/mo.

If it doesn’t work without a middleman then it doesn’t make sense to run your own service. Any spammer can rent a server the way you can.

That's their cost for a dedicated "clean" transit. At $15/mo you're just sharing tenancy with everyone else, using their SMTP setup together with everyone else. I'm running my own setup for a handful of domains, my own configurations, signing procedures, filters, features, aliases and catchalls etc. Incidentally almost all spam I receive comes in via Gmail, Yahoo and Hotmail.

What matters is how inbox providers look at the source IP. At least delivery services have higher chance by filtering obvious outgoing spam than a rogue server IP.

My problem isn't the network I'm originating from. It's a) that Proofpoint doesn't track "state" (message-IDs) between outbound and returning e-mail, and b) that I don't send regularly enough to be a "familiar".