I also moved to a DigitalOcean VPS, their lack of documentation regarding sendmail, postfix, exim and other "send email" solutions is really disappointing!
WP Mail SMTP plugin has been as must have to some of my WP web sites for a long time, but after moving to DO I'm completely lost, I think they suppose everyone is meant to be an expert sysadmin and for this reason their support is minimal.
I'm not complaining about DO services, their VPS's are great, my wordpress sites are blowing fast, you get a high performance VPS for a fair price and so on, but I'm stuck, and DO community support is not able to help either. I'm now considering moving on to a managed WP service such as wp-engine. So sad!