It largely depends on where you’re hosting your mail server. Certain providers (e.g. Digital Ocean) are a complete no-go. Their IP ranges are completely untrusted.
DigitalOcean blocks port 25 now anyway[0], since I think June this year, for all new accounts. So not really a viable option now. We have to relay email from our servers there. Bit annoying, and no support won't make an exception or turn it off after a bit of good behaviour.