[nicolast]

Exactly: a non-blacklisted IP with good reputation, correct mailname and valid PTR record is the foundation, ideally augmented with SPF. I've been running an e-mail service without DKIM and DMARC for many years without any delivery issues, only adding those very recently.

[Avamander]

Ideally? No. SPF (or DKIM) is pretty much mandatory for delivering mail.