Fair. To a certain mindset, "self-hosted" is its own advantage -- but to pick a couple of advantages out of thin air:
+ Free: I can run Postal on my own hardware, with no usage limits or costs other than the VPS or AWS costs (which I'm paying anyway)
+ Independent: I'm not relying on a third-party to provide core features. If SendGrid goes down there's nothing I can do about it
+ Private: Maybe I don't want to hand over a list of my user's email addresses to SendGrid.
Self-hosting services means I maintain control over them. It's a tradeoff, and you may value control less than, say, convenience. YMMV.
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.
Imagine a retail business that relies on emails to get information on what's next: what are the most popular products, what's a good tag line, which segment of its customers are interested by what, who is most likely to buy. Sendgrid allows you to add A/B testing, scheduling (send a % of emails at a specific hour), track who opened emails when, track who unsubsucribed.
If you are doing simple emails like email confirmation or simple user reminder, these features are totally overkill. But email has still the best ROI for many businesses and thats why they need sendgrid
Assuming it has an API, I would prefer to use this over a service such as Mailgun. We used to have our own mail server, but without an easy API to get deliverability details per mail, we ended up switching to Mailgun. Since the switch, our deliverability has nosedived (we're currently not paying for a dedicated IP though) and mails are constantly bouncing due to the low reputation of Mailgun's shared IPs.
Mailgun's 30-day retention period is also a bit absurd, since most of our clients require mail audit trails in the region of 6-12 months.
Some organizations can't (due to regulation) or won't (due to privacy/security concerns) give their email list to third parties. That eliminates most of the third party MTAs as they all maintain sending lists, logs etc.
