I feel like you're still begging the question. The proposition is not really "SendGrid vs Self-hosted SendGrid", but "Self-hosted SendGrid vs any IMAP/SMTP server":
> Free: I can run Postal on my own hardware, with no usage limits or costs other than the VPS or AWS costs.
So is postfix.
> Independent: I'm not relying on a third-party to provide core features.
What features? This is the crux of the question. What do you actually need that justifies having to maintain a web application?
> Private: Maybe I don't want to hand over a list of my user's email addresses to SendGrid.
> Free: I can run Postal on my own hardware, with no usage limits or costs other than the VPS or AWS costs.
So is postfix.
> Independent: I'm not relying on a third-party to provide core features.
What features? This is the crux of the question. What do you actually need that justifies having to maintain a web application?
> Private: Maybe I don't want to hand over a list of my user's email addresses to SendGrid.
Again... so is Postfix.