Interesting; my general experience (and that of customers I spent time working with doing support for various cloud providers) was that you could, theoretically do so, but 'sending the email to a provider' and 'the provider accepts it and delivers it' were not always the same thing.

Microsoft was especially bad in that it would accept the message, and give you the standard SMTP 'message accepted' response but then silently just drop it in the backend, never to be seen again. Didn't go to spam, didn't land in a filter just... vanished.

Google, at least, had the decency to tell you when it was going to reject your email, but still.

It was always the same dance: you need a PTR, an SPF record, DKIM, etc. but at the end of the day, Google and Microsoft absolutely gatekeep what gets delivered to their platform, so if it's critical that your email shows up reliably every time, you have to move into the "ecosystem" of ESPs and all the hoops that are involved there if you want your message to go to the 'big providers'.