As others have said, this particular service uses Amazon. But to answer your question, I've supported services where management dictated the process was going to involve an on-premises Exchange server built just for a mail campaign to deliver directly to inboxes.
The answer is, rDNS, SPF, DKIM, abuse.net registry, feedback loops with major mail services - are all memes.
You can do everything right, which is an extremely manual effort once you get as far as building code that identifies the top mail services used by your mailing list and reaching out to ISPs and mail hosts individually and asking them to whitelist you.
And you'll still find deliverability extremely poor.
Even Troy Hunt went in circles trying to get HIBP emails delivered to Microsoft Office 365 users.
The answer is, rDNS, SPF, DKIM, abuse.net registry, feedback loops with major mail services - are all memes.
You can do everything right, which is an extremely manual effort once you get as far as building code that identifies the top mail services used by your mailing list and reaching out to ISPs and mail hosts individually and asking them to whitelist you.
And you'll still find deliverability extremely poor.
Even Troy Hunt went in circles trying to get HIBP emails delivered to Microsoft Office 365 users.