FWIW running a service who's job is to send email, I have found that GMail is one of the best providers to send to. They strongly value domain reputation and are popular enough that it was quick to build up a basic reputation. Now my messages are rarely flagged as spam even coming from a Digital Ocean IP address. I just rely on DKIM.

This is in contrast to Microsoft which seems to rely mostly on IP reputation and trying to send from a new server is incredibly difficult as they (reasonably) treat unknown Digital Ocean IPs as likely spam but (unreasonably) don't allow a good domain reputation to override that.