[cnst]

But how would anyone know it's Gmail or Apple if the IP address is new?

That's exactly my point, that the reputation need is overstated by all those services that claim to solve a known problem that everyone has heard of, but noone has actually experienced, because, guess what, it might not actually exist.

I've seen plenty of cases where the emails sent out through Sendgrid et al, end up in the Spam folder, or these "professional" services don't even attempt to retry, thus, never getting through the greylisting, or other bugs which cause deliverability issues, which would never happen if you were to run your own real mail-server on your own hardware yourself.

[SoftTalker]

Sendgrid, Mailchimp, et. al. ends up in the spam folder because most of what they send is spam.

[cnst]

I'm not disputing that assertion, yet it does go against the marketing materials we're all presented by all of these services, as for reasons to not run our own mailservers.

In other words, if all you want to do is run a personal mailserver, or even a corporate one, you'll probably not have to deal with this supposed IP reputation issue, unless the IP addresses you use, have already been added to the blacklists even before you start at it.