Summarizing the relevant parts of an eerily similar conversation I had the other day:

If you are using the built-in mail relay then you aren’t signing your mail with DKIM, don’t have SPF set up right, don’t have a DMARC policy, and don’t have FcRDNS, all of which basically any mail provider will require from you to even consider accepting your mail. Basically without all of that literally anyone can pretend to be whatever.com and send email from it. They really shouldn’t be shipping that mail relay at all IMO, it just leads to confusion. More than likely you would already know if you need a mail relay and be able to set it up yourself if so.

Sendgrid and Postmark are popular transactional mail services (which is what sort of email you will be sending, google that term to find more options). If you want some help getting your own mail server set up in a dockerized way I run my mail using docker-mailserver and if only set up for outgoing mail it is pretty easy to run, though you will probably run into deliverability issues as the large providers (google, microsoft, apple, etc) can be real assholes and assume anything from a non-large provider is spam. Feel free to ask me about how to do it if you are interested though, the more people run their own mail the better it gets for all of us.