Many services (including postfix by default) will attempt a number of resend operations before it gives up.
Of course, federated email does not use SPF/DKIM/DMARC because the whole point is that someone from another server could use your server to send an email (hence the federation).
What? All email is federated. What are you talking about here? SPF/DKIM/DMARC are on top of email… and have nothing to do with the federated property of email. Federation does not mean that you login or use another server. But that you have your instance, and the servers hash out the cross communication amongst themselves. That’s EXACTLY what email servers do using SMTP.
I would definitely set it up securely and not as a federated server otherwise it would be practically unusable for day to day emails.
If your email wasn’t federated then you would get emails from anyone outside of your own instance. That would make email useless for 99% of the world.