I've been running my mail server off DO for the last ±5 years - Postfix with SPF no DKIM, DMARC - I get good delivery for Gmail and yahoo. Initially, I thought I would go full on spamassassin and clamav but thought I should hold off until I got spammed. Turns out in the last 5 years the only spammers I've got are 2 senders, one threatening to share video recorded of me on webcam to my contacts and another keeps telling me my domains' seo has expired blah blah. These are 1 or 2 times in 3 months, SpamAssassin would definitely be overkill. Apple Mail client junks them anyway. About 1 year ago I moved the setup into a docker container to better maximize the use of the $10 droplet.