I can't say I'd recommend DO for an email server either. I wasn't blacklisted per se, I was blocked before I even sent any email and needed to be whitelisted.

I won't say DO is a bad choice for a mailserver, nor a good one. I just did a quick search to see if that's changed in the past few years and came across "Helm", but that's not really a "good" option either.

The real problem is there is no "good" choice for a dedicated mailserver or service provider.