It's become popular for some providers since the move to the Cloud to block outgoing port 25, but apart from that, you literally don't need Sendgrid. Any senior engineer can write a local "Sendgrid" in about half a day, if not less. Sendgrid doesn't even support all the RFCs properly, so, you'll likely get dropped mail if you move to Sendgrid from Postfix/Sendmail. Besides that, Sendgrid's UI/UX was terrible the last time I've played with it. It's literally the lamest startup there could possibly be.
As a customer of some unrelated service, I've already converted said service away from Sendgrid in just a couple of emails to the owner. (Sadly, they've moved to one of the many Sendgrid's competitors, not to Postfix/Sendmail, but the switch was obviously painless, and actually improved reliability.) That's how horrible Sendgrid is, and how easy it is to make other people switch.
P.S. I often send mail through shell scripts. Try selling me Sendgrid when my laptop already comes with Postfix/Sendmail, with spool support. Does Sendgrid even work when you have no internet connection? (That was a rhetorical question; the answer is, No, Sendgrid does not work when internet is not available; but Postfix/Sendmail works just fine even without any internet at all, and Postfix/Sendmail will spool all your mail until you get connected to an internet with port 25 not blocked.) Sendgrid is a downgrade however you look at it. When they move to MFA, it'll probably be easier to just switch to one of their plentiful competitors than deal with any new restrictions; that's why Sendgrid will probably be very careful around any new requirements; because their service is 100% disposable.
You seem to not understand the sendgrid service at all. They aren't selling SMTP service, they are selling deliverability (along with tracking and metrics)
You seem to not understand the actual problem at all if you think you will be able to successfully deliver email sent directly from a local sendmail client on any random IP that lets you connect over port 25. No major email provider will accept your email coming from a residential IP.
If that's what they're selling, they're not very good at it. Sendgrid doesn't support all applicable RFCs, and they don't support spool, so, they have exactly 0% deliverability if the recipient host is down for any reason, or uses greylisting.
I've never had any issues with mail from my WiFi IP accepted. You'd normally send it just to your own server from your local laptop w/o any extra configuration, so, it's up to you how you configure your own server. I just tried sending to Gmail directly from Postfix on my MacBook, without any extra configuration, and it's been accepted by Gmail as well. Zero configuration on the laptop. Postfix and the mail CLI clients come preinstalled, and the mail just works.
You're mainly paying to deal with deliverability (IP reputation, proper "ramping" of mail servers, and the increasing number of technical requirements for correctly operating a mail server).
It's become popular for some providers since the move to the Cloud to block outgoing port 25, but apart from that, you literally don't need Sendgrid. Any senior engineer can write a local "Sendgrid" in about half a day, if not less. Sendgrid doesn't even support all the RFCs properly, so, you'll likely get dropped mail if you move to Sendgrid from Postfix/Sendmail. Besides that, Sendgrid's UI/UX was terrible the last time I've played with it. It's literally the lamest startup there could possibly be.
As a customer of some unrelated service, I've already converted said service away from Sendgrid in just a couple of emails to the owner. (Sadly, they've moved to one of the many Sendgrid's competitors, not to Postfix/Sendmail, but the switch was obviously painless, and actually improved reliability.) That's how horrible Sendgrid is, and how easy it is to make other people switch.
P.S. I often send mail through shell scripts. Try selling me Sendgrid when my laptop already comes with Postfix/Sendmail, with spool support. Does Sendgrid even work when you have no internet connection? (That was a rhetorical question; the answer is, No, Sendgrid does not work when internet is not available; but Postfix/Sendmail works just fine even without any internet at all, and Postfix/Sendmail will spool all your mail until you get connected to an internet with port 25 not blocked.) Sendgrid is a downgrade however you look at it. When they move to MFA, it'll probably be easier to just switch to one of their plentiful competitors than deal with any new restrictions; that's why Sendgrid will probably be very careful around any new requirements; because their service is 100% disposable.