SMTP is blocked on DO by default. If you want to use mail, you’ll need to find a proxy somewhere. Lots of people have said that Mailgun is what works good for Discourse on the cloud but that is not something I have experience with.
This might be what you’re after:
Yes, you can use social login (Discord, Google, …) or Discourse ID (no setup) without SMTP. You’ll lose digests and email notifications, but if your goal is just a community it will work fine even with lower engagement (digests are designed to help retention).